What airlines fly to Chicago O'Hare Intl Airport?

United Airlines, American Airlines and Frontier are the top three most popular airlines that fly to Chicago O'Hare Intl Airport from Baltimore according to our data. United Airlines is the first choice among our users, with 57% of people opting to fly with this airline when traveling to Chicago O'Hare Intl Airport. Keep in mind that there are 5 different airlines that fly to Chicago O'Hare Intl Airport on a routine basis, so you have a few additional options to choose from.

How many flights depart from Baltimore to Chicago O'Hare Intl Airport on average per day?

As of December 2025, there are 9 flights per day flying from Baltimore to Chicago O'Hare Intl Airport. The most common time of day flights depart is 6:00AM, with 63% of flights taking off at noon.

What is the cheapest airline that flies from Baltimore/Washington to Chicago O'Hare Intl?

United Airlines is the most popular carrier operating from Baltimore/Washington to Chicago O'Hare Intl

How many flights depart from Baltimore to Chicago O'Hare Intl Airport on average per week?

As of December 2025, 67 flights take off from Baltimore and land in Chicago O'Hare Intl Airport each week. The most common flight departure day for this route is Wed, with 24% of weekly flights taking off on this day.

When did momondo last update prices for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport?

Prices for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport refresh daily.

How many operators does momondo search for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport?

As of December 2025, there are 5 operators available to choose from for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport.

How old do you have to be to fly from Baltimore/Washington Airport to Chicago O'Hare Intl Airport?

The minimum age for a child to fly alone is five, but airlines offering an unaccompanied minor (UNMR) service may have their age restrictions, such as regulations concerning flight duration, timing, and layovers. It is recommended that you verify with the airline you are booking with for travel from Baltimore/Washington Airport to Chicago O'Hare Intl Airport.

What documentation or ID do you need to fly to Chicago O'Hare Intl Airport?

The required travel and health documents for your Chicago O'Hare Intl Airport trip may vary depending on your itinerary and personal circumstances. As a general rule, you should carry multiple documents, including a national ID card or driver's license and a passport valid for six months after your intended arrival at Chicago O'Hare Intl Airport. We recommend that you seek guidance from your airline or a reliable third party such as IATA.

$67 Cheap Flights from Baltimore to Chicago O'Hare Intl Airport in 2025/26

1 adult

0 bags

Discover deals from 900+ travel sites with momondo.

Save money when you book flights with momondo

Big names and deals

Search 100s of travel sites to compare prices.

Filter for what you want

Free Wi-Fi? Stopover? Instantly customize your results.

Trusted and free

We’re completely free to use - no hidden charges or fees.

Price Alerts

Not ready to book? to track prices.

Search flight deals

Find the cheapest flights from Baltimore to Chicago

Round-trip

One-way

Last minute

Direct

Cheap round-trip flights

Enter your travel dates to find the best prices.

11:30 am - 7:45 pmBWI-ORD

9h 15m1 stop

5:01 am - 8:03 amORD-BWI

2h 02mnonstop

$99Multiple Airlines

Find Deal

Sat, Feb 21 - Sun, Feb 22

8:17 pm - 9:41 pmBWI-ORD

2h 24mnonstop

7:30 am - 8:31 amORD-BWI

24h 01m1 stop

$101Multiple Airlines

Find Deal

Fri, Jan 9 - Sun, Jan 11

11:30 am - 5:52 pmBWI-ORD

7h 22m1 stop

10:48 am - 11:47 pmORD-BWI

11h 59m1 stop

$102Spirit Airlines

Find Deal

Tue, Jan 13 - Wed, Jan 14

8:38 pm - 9:48 pmBWI-ORD

2h 10mnonstop

5:57 pm - 12:31 amORD-BWI

5h 34m1 stop

$103Multiple Airlines

Find Deal

Wed, Jan 7 - Tue, Jan 13

8:17 pm - 9:41 pmBWI-ORD

2h 24mnonstop

7:30 am - 8:31 amORD-BWI

24h 01m1 stop

$104Multiple Airlines

Find Deal

Fri, Jan 9 - Sun, Jan 11

8:17 pm - 9:41 pmBWI-ORD

2h 24mnonstop

4:20 pm - 7:22 pmORD-BWI

2h 02mnonstop

$105Frontier

Find Deal

Fri, Jan 9 - Sun, Jan 11

8:07 pm - 9:31 pmBWI-ORD

2h 24mnonstop

12:44 pm - 11:47 pmORD-BWI

10h 03m1 stop

$106Multiple Airlines

Find Deal

Thu, Jan 8 - Mon, Jan 12

3:36 pm - 4:54 pmBWI-ORD

2h 18mnonstop

5:11 am - 8:18 amORD-BWI

2h 07mnonstop

$107Frontier

Find Deal

Mon, Dec 8 - Sun, Dec 14

6:18 pm - 8:59 pmBWI-ORD

27h 41m1 stop

4:20 pm - 7:22 pmORD-BWI

2h 02mnonstop

$108Multiple Airlines

Find Deal

Tue, Jan 6 - Thu, Jan 8

8:00 am - 2:31 pmBWI-ORD

7h 31m1 stop

5:11 am - 8:18 amORD-BWI

2h 07mnonstop

$109Frontier

Find Deal

Wed, Dec 10 - Sun, Dec 14

Frontier flights from Baltimore to Chicago O'Hare Intl Airport

Our best Frontier deals on Baltimore to Chicago O'Hare Intl Airport flight tickets

Frontier

Jan 9

BWI8:17 pmBaltimore/Washington

2h 24m

ORD9:41 pmChicago O'Hare Intl

Jan 11

ORD4:20 pmChicago O'Hare Intl

2h 02m

BWI7:22 pmBaltimore/Washington

$105

Find similar

Frontier

Dec 10

BWI8:00 amBaltimore/Washington

7h 31m

1 stop

ORD2:31 pmChicago O'Hare Intl

Dec 14

ORD5:11 amChicago O'Hare Intl

2h 07m

BWI8:18 amBaltimore/Washington

$106

Find similar

Frontier

Jan 8

BWI8:07 pmBaltimore/Washington

2h 24m

ORD9:31 pmChicago O'Hare Intl

Jan 11

ORD8:00 amChicago O'Hare Intl

22h 21m

1 stop

BWI7:21 amBaltimore/Washington

$107

Find similar

Spirit Airlines flights from Baltimore to Chicago O'Hare Intl Airport

Our best Spirit Airlines deals on Baltimore to Chicago O'Hare Intl Airport flight tickets

Spirit Airlines

Jan 13

BWI11:30 amBaltimore/Washington

7h 22m

1 stop

ORD5:52 pmChicago O'Hare Intl

Jan 14

ORD10:48 amChicago O'Hare Intl

11h 59m

1 stop

BWI11:47 pmBaltimore/Washington

$102

Find similar

Spirit Airlines

Dec 14

BWI10:40 amBaltimore/Washington

12h 08m

1 stop

ORD9:48 pmChicago O'Hare Intl

Jan 10

ORD12:44 pmChicago O'Hare Intl

10h 03m

1 stop

BWI11:47 pmBaltimore/Washington

$135

Find similar

Spirit Airlines

Dec 11

BWI8:45 amBaltimore/Washington

12h 16m

1 stop

ORD8:01 pmChicago O'Hare Intl

Dec 12

ORD12:09 pmChicago O'Hare Intl

11h 14m

1 stop

BWI12:23 amBaltimore/Washington

$161

Find similar

momondo Fare Finder

Use the filters below to find and compare flights from Baltimore to Chicago O'Hare Intl Airport that are best for you.

Fly from

Fly to

Route

Depart

Return

Price

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Sat 2/21

11:30 am-7:45 pm

1 stop9h 15m

Chicago O'Hare Intl

Sun 2/22

5:01 am-8:03 am

Nonstop2h 02m

$99

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Fri 1/9

8:17 pm-9:41 pm

Nonstop2h 24m

Chicago O'Hare Intl

Sun 1/11

7:30 am-8:31 am

1 stop24h 01m

$101

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Tue 1/13

11:30 am-5:52 pm

1 stop7h 22m

Chicago O'Hare Intl

Wed 1/14

10:48 am-11:47 pm

1 stop11h 59m

$102

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Wed 1/7

8:38 pm-9:48 pm

Nonstop2h 10m

Chicago O'Hare Intl

Tue 1/13

5:57 pm-12:31 am

1 stop5h 34m

$103

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Fri 1/9

8:17 pm-9:41 pm

Nonstop2h 24m

Chicago O'Hare Intl

Sun 1/11

7:30 am-8:31 am

1 stop24h 01m

$104

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Fri 1/9

8:17 pm-9:41 pm

Nonstop2h 24m

Chicago O'Hare Intl

Sun 1/11

4:20 pm-7:22 pm

Nonstop2h 02m

$105

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Thu 1/8

8:07 pm-9:31 pm

Nonstop2h 24m

Chicago O'Hare Intl

Mon 1/12

12:44 pm-11:47 pm

1 stop10h 03m

$106

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Mon 12/8

3:36 pm-4:54 pm

Nonstop2h 18m

Chicago O'Hare Intl

Sun 12/14

5:11 am-8:18 am

Nonstop2h 07m

$107

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Tue 1/6

6:18 pm-8:59 pm

1 stop27h 41m

Chicago O'Hare Intl

Thu 1/8

4:20 pm-7:22 pm

Nonstop2h 02m

$108

BaltimoreChicago

BWI - ORD

BWIORD

Baltimore/Washington

Wed 12/10

8:00 am-2:31 pm

1 stop7h 31m

Chicago O'Hare Intl

Sun 12/14

5:11 am-8:18 am

Nonstop2h 07m

$109

Flights are sorted by cheapest round-trip flights first.

Showing 1-10 of 20 resultsSorted by cheapest first

How to get the cheapest flight ticket from Baltimore to Chicago O'Hare Intl Airport

Look no further. We've gone through all the searches for this route on momondo so you have the important information and insights to find the cheapest flight ticket for your trip

Prices

Airlines

Other information

What is the cheapest month to fly to Chicago O'Hare Intl Airport?

The month of January is another great option to travel to Chicago O'Hare Intl Airport, with average flight prices from Baltimore averaging around $232.

What is the cheapest day to depart on your flight to Chicago O'Hare Intl Airport?

The cheapest day to depart on your flight to Chicago O'Hare Intl Airport is on a Friday when prices can be found for as low as $387. This is often the case because there is less demand to fly during this day of the week. On the other hand, Sunday is the most expensive day to depart from Baltimore because it is more convenient for both business and leisure travelers. As a result, prices tend to be around $528.

What is the cheapest time of day to depart on your flight to Chicago O'Hare Intl Airport?

Flying to Chicago O'Hare Intl Airport from Baltimore in the evening is the least expensive time of day to depart. momondo users have found prices as low as $354 when looking at flights departing in the evening. Try not to purchase tickets in the afternoon because they tend to be around $449. Of course, these prices can vary depending on the time of year and what airport you fly to.

What is the cheapest Baltimore/Washington to Chicago O'Hare Intl flight route?

Fly from Baltimore/Washington to Chicago O'Hare Intl for the best Baltimore/Washington - Chicago O'Hare Intl flight prices

How far in advance should I book a flight to Chicago O'Hare Intl Airport?

You are more likely to see cheaper flights to Chicago O'Hare Intl Airport from Baltimore when booking weeks ahead of your departure date. In general, you can expect to find flights to Chicago O'Hare Intl Airport for around $282 when searching weeks out. However, flight prices might be lower outside of this timeframe. For example, people recently found flights to Chicago O'Hare Intl Airport from Baltimore for as low as $226 on momondo.

What cabin class options are available on Baltimore/Washington to Chicago O'Hare Intl flights?

There are 4 cabin classes available to choose from on Baltimore/Washington to Chicago O'Hare Intl flights. Cabin price and availability will vary by airline so perform a search above to find the right deals for you.

How have Baltimore to Chicago O'Hare Intl Airport flight prices changed over time?

As of December 2025, prices for Baltimore to Chicago O'Hare Intl Airport flights are approximately 9% cheaper when compared to flights at the same time last year.

When to book flights from Baltimore to Chicago O'Hare Intl Airport

Are you open to changing your travel dates? Check out the ideal times to fly from Baltimore to Chicago O'Hare Intl Airport according to our flight data. Find out which month and days have the cheapest flights.

Estimated round-trip price

Cheap Baltimore to Chicago O'Hare Intl Airport (BWI to ORD) flight deals and tips

Find info about flight duration, direct flights, and airports for your flight from Baltimore to Chicago O'Hare Intl Airport

Best Price Found	$97
Fastest flight time	2h 10m
Direct flights	Every day

The best one-way flight to Chicago O'Hare Intl Airport from Baltimore in the past 72 hours is $97. The best round-trip flight deal from Baltimore to Chicago O'Hare Intl Airport found on momondo in the last 72 hours is $198.

The fastest flight from Baltimore to Chicago O'Hare Intl Airport takes 2h 10m

Direct flights go from Baltimore to Chicago O'Hare Intl Airport every day.

Search flight deals

How far is Baltimore/Washington to Chicago O'Hare Intl by plane?
The straight-line distance from Baltimore/Washington to Chicago O'Hare Intl is 619.6 mi. By plane, this route typically takes 2h 10m.
When did momondo last update prices for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport?
Prices for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport refresh daily.
How many operators does momondo search for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport?
As of December 2025, there are 5 operators available to choose from for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport.
How old do you have to be to fly from Baltimore/Washington Airport to Chicago O'Hare Intl Airport?
The minimum age for a child to fly alone is five, but airlines offering an unaccompanied minor (UNMR) service may have their age restrictions, such as regulations concerning flight duration, timing, and layovers. It is recommended that you verify with the airline you are booking with for travel from Baltimore/Washington Airport to Chicago O'Hare Intl Airport.
What documentation or ID do you need to fly to Chicago O'Hare Intl Airport?
The required travel and health documents for your Chicago O'Hare Intl Airport trip may vary depending on your itinerary and personal circumstances. As a general rule, you should carry multiple documents, including a national ID card or driver's license and a passport valid for six months after your intended arrival at Chicago O'Hare Intl Airport. We recommend that you seek guidance from your airline or a reliable third party such as IATA.
What is the most popular plane model from Baltimore to Chicago O'Hare Intl Airport?
Based on momondo bookings for this flight route, the most popular plane model on flight routes from Baltimore to Chicago O'Hare Intl Airport is Boeing 737-900 (9.49% of total bookings). The next most popular plane models are Airbus A319 (8.91%), Airbus A320-100/200 (8.75%), and Embraer 175 (8.36%).
What is the most popular airline from Baltimore to Chicago O'Hare Intl Airport?
momondo data reveals that the most frequently booked airline on the route from Baltimore to Chicago O'Hare Intl Airport is United Airlines, representing 43% of all bookings. The subsequent popular airlines are American Airlines at 23%, Frontier at 16%, and Southwest at 13%.

Best airlines flying from Baltimore to Chicago O'Hare Intl Airport

Compare and see reviews for airlines that fly from Baltimore to Chicago O'Hare Intl Airport with momondo

Airline	Rating	Free Cancellation	Price
Spirit Airlines	PriceFrom $52	Free Cancellation	Rating 6.3 8,156 reviews	Search Spirit Airlines flights
Frontier	PriceFrom $67	Free Cancellation	Rating 5.8 3,094 reviews	Search Frontier flights
Southwest	PriceFrom $150	Free Cancellation	Rating 8.0 4,253 reviews	Search Southwest flights
United Airlines	PriceFrom $158	Free Cancellation	Rating 7.4 9,063 reviews	Search United Airlines flights
American Airlines	PriceFrom $168	Free Cancellation	Rating 7.2 11,045 reviews	Search American Airlines flights

United Airlines Flights From Baltimore to Chicago O'Hare Intl Airport
United Airlines is one of the most popular airlines flying from Baltimore to Chicago O'Hare Intl Airport, operating 14 flights/week on average for this route. A good United Airlines flight deal for Baltimore to Chicago O'Hare Intl Airport starts from $158 round-trip, up to an average price of $309. Our users have rated United Airlines at 7.4/10.
American Airlines Flights From Baltimore to Chicago O'Hare Intl Airport
American Airlines (7.2/10 from 11,045 reviews) operates around 8 flights from Baltimore to Chicago O'Hare Intl Airport in any given week. The average American Airlines airfare for this route is around $317, with deals from around $168 rare but found on momondo. Referenced prices are for round-trip Baltimore to Chicago O'Hare Intl Airport flight searches in the last 72 hours.

Lake Geneva

South Haven

Schaumburg

Rockford

Merrillville

Gurnee

Naperville

Rosemont

Kenosha

New Buffalo

Joliet

Oak Brook

Orland Park

Racine

Hammond

Tinley Park

Janesville

Aurora

Elgin

Benton Harbor

Why use momondo?

Why should I use momondo to find a flight deal from Baltimore to Chicago?

momondo collates all your flight options in one place and lets you filter them to find the perfect flight from Baltimore to Chicago. momondo saves you time and could save you money on your trip.

Can I find flights from Baltimore to Chicago with flexible booking policies on momondo?

Yes. momondo enables you to select and highlight flights to Chicago from Baltimore with this policy.

How can momondo help me find flights from Baltimore to Chicago?

Once all potential Baltimore to Chicago flight options are in front of you, momondo lets you sort and filter flights based on preferences such as airline, price and flight times amongst many other options.

I’m not ready to book a flight from Baltimore to Chicago right now. Can momondo alert me if prices change?

Yes momondo offers price alerts on flights to Chicago from Baltimore. After performing a search on this page you should be able to see the price alert set up feature. Simply provide a valid email address and momondo will instantly alert you when prices change.

An easier way to manage your Chicago O'Hare Intl Airport trip

We make it super easy to schedule, organize and travel with friends or family. Trips is free — and available to use no matter where you book.

Start using Trips

Find my bookings

Get notified when prices go down

Daily price changes in your inbox - only with Price Alerts.

1 adult

Fri 1/2

Fri 1/9

Email me momondo's favorite deals

By creating a price alert you accept our terms of use and privacy policy.

Find better results for your trip to Chicago

Popular cities in the United States

Flights to Newark Airportfrom $49

Flights to New York John F Kennedy Intl Airportfrom $39

Flights to New York LaGuardia Airportfrom $53

Flights to Dallas/Fort Worth Airportfrom $53

Flights to Washington, D.C. Dulles Intl Airportfrom $54

Flights to Orlando Airportfrom $41

Flights to Washington, D.C. Reagan-National Airportfrom $73

Flights to Chicago Midway Airport

Flights to Dallas Love Field Airport

Cheap flights from Baltimore to Chicago from $67

This is the cheapest one-way flight price found by a momondo user in the last 72 hours by searching for a flight from Baltimore to Chicago departing on 1/30. Fares may change, and may not be available for all flights or travel dates. Click the price to refresh the search for this deal.

Big names and deals

Big names and deals

Filter for what you want

Filter for what you want

Trusted and free

Trusted and free

Price Alerts

Price Alerts

Find the cheapest flights from Baltimore to Chicago

Cheap round-trip flights

Cheap one-way flights

Cheap last minute flights

Cheap direct flights

Frontier flights from Baltimore to Chicago O'Hare Intl Airport

Our best Frontier deals on Baltimore to Chicago O'Hare Intl Airport flight tickets

Spirit Airlines flights from Baltimore to Chicago O'Hare Intl Airport

Our best Spirit Airlines deals on Baltimore to Chicago O'Hare Intl Airport flight tickets

momondo Fare Finder

Use the filters below to find and compare flights from Baltimore to Chicago O'Hare Intl Airport that are best for you.

See travel insights to Chicago from

How to get the cheapest flight ticket from Baltimore to Chicago O'Hare Intl Airport

Look no further. We've gone through all the searches for this route on momondo so you have the important information and insights to find the cheapest flight ticket for your trip

What is the cheapest month to fly to Chicago O'Hare Intl Airport?

The month of January is another great option to travel to Chicago O'Hare Intl Airport, with average flight prices from Baltimore averaging around $232.

What is the cheapest day to depart on your flight to Chicago O'Hare Intl Airport?

What is the cheapest time of day to depart on your flight to Chicago O'Hare Intl Airport?

What is the cheapest Baltimore/Washington to Chicago O'Hare Intl flight route?

Fly from Baltimore/Washington to Chicago O'Hare Intl for the best Baltimore/Washington - Chicago O'Hare Intl flight prices

How far in advance should I book a flight to Chicago O'Hare Intl Airport?

What cabin class options are available on Baltimore/Washington to Chicago O'Hare Intl flights?

There are 4 cabin classes available to choose from on Baltimore/Washington to Chicago O'Hare Intl flights. Cabin price and availability will vary by airline so perform a search above to find the right deals for you.

How have Baltimore to Chicago O'Hare Intl Airport flight prices changed over time?

As of December 2025, prices for Baltimore to Chicago O'Hare Intl Airport flights are approximately 9% cheaper when compared to flights at the same time last year.

What is the cheapest airline that flies from Baltimore/Washington to Chicago O'Hare Intl?

United Airlines is the most popular carrier operating from Baltimore/Washington to Chicago O'Hare Intl

What airlines fly to Chicago O'Hare Intl Airport?

How many flights depart from Baltimore to Chicago O'Hare Intl Airport on average per day?

As of December 2025, there are 9 flights per day flying from Baltimore to Chicago O'Hare Intl Airport. The most common time of day flights depart is 6:00AM, with 63% of flights taking off at noon.

How many flights depart from Baltimore to Chicago O'Hare Intl Airport on average per week?

As of December 2025, 67 flights take off from Baltimore and land in Chicago O'Hare Intl Airport each week. The most common flight departure day for this route is Wed, with 24% of weekly flights taking off on this day.

When to book flights from Baltimore to Chicago O'Hare Intl Airport

Cheap Baltimore to Chicago O'Hare Intl Airport (BWI to ORD) flight deals and tips

Find info about flight duration, direct flights, and airports for your flight from Baltimore to Chicago O'Hare Intl Airport

Best Price Found

Fastest flight time

Direct flights

How far is Baltimore/Washington to Chicago O'Hare Intl by plane?

When did momondo last update prices for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport?

How many operators does momondo search for flights from Baltimore/Washington Airport to Chicago O'Hare Intl Airport?

How old do you have to be to fly from Baltimore/Washington Airport to Chicago O'Hare Intl Airport?

What documentation or ID do you need to fly to Chicago O'Hare Intl Airport?

What is the most popular plane model from Baltimore to Chicago O'Hare Intl Airport?

What is the most popular airline from Baltimore to Chicago O'Hare Intl Airport?

Best airlines flying from Baltimore to Chicago O'Hare Intl Airport

Compare and see reviews for airlines that fly from Baltimore to Chicago O'Hare Intl Airport with momondo

United Airlines Flights From Baltimore to Chicago O'Hare Intl Airport

American Airlines Flights From Baltimore to Chicago O'Hare Intl Airport

Traveling to a place near Chicago?

Why use momondo?

Why should I use momondo to find a flight deal from Baltimore to Chicago?

Can I find flights from Baltimore to Chicago with flexible booking policies on momondo?

How can momondo help me find flights from Baltimore to Chicago?

I’m not ready to book a flight from Baltimore to Chicago right now. Can momondo alert me if prices change?

An easier way to manage your Chicago O'Hare Intl Airport trip

We make it super easy to schedule, organize and travel with friends or family. Trips is free — and available to use no matter where you book.

Get notified when prices go down

Daily price changes in your inbox - only with Price Alerts.

Find better results for your trip to Chicago

Popular cities in the United States

Popular flights according to clicks in the last 7 days on momondo