The new gates are part of the city's 2025 gate reallocation process.
O'Hare's gates are reallocated based on how many flights each carrier flew in the prior year.
Under the new gate layout, United will have 95 gates. American Airlines will have 59 gates.
Last month, American Airlines filed a lawsuit against the city to try to stop United from getting the additional gates.
At that time, United said American has occupied 41% of O'Hare's gates, while only operating 37% of departures from the airport.
READ ALSO | American, United airlines battle over gates at O'Hare airport
A spokesperson for American Airlines released the following statement:
"The Chicago Department of Aviation's (CDA) premature trigger of the gate reallocation at Chicago O'Hare International Airport (ORD) upsets the competitive balance at O'Hare, especially at a time when American is operating its biggest summer on record with 25% more seats compared to last year and the addition of 18 new destinations for customers to enjoy in 2025."
"We recently took legal action against the CDA's violation of the 2018 Airlines Use and Lease agreement (AULA), which was carefully negotiated to prevent this very issue at hand - by outlining important benchmarks that must be met before the reallocation was triggered in order to ensure that Chicagoans continue to benefit from competition."