In exchange, the Thunder agreed to send centers Al Horford and Moses Brown and a 2023 second-round pick to the Celtics.
Walker, 31, joined the Celtics in a 2019 trade from the Charlotte Hornets. The four-time All-Star averaged 19.3 points, 4.9 assists and four rebounds per game in 43 starts this season. Walker battled injuries throughout the last two seasons.
Walker is under contract for the next two seasons, with a $37.6 million player option in 2022-23. His $34.3 million salary in 2020-21 was the highest among Celtics players.
Horford is under contract for the next two seasons. Brown is signed through 2022-23, with a team option in 2023-24.
Horford, 35, averaged 14.2 points, 6.7 rebounds and 3.4 assists in 28 starts this season for the Thunder. He joined the team in a December trade from the Philadelphia 76ers.
The five-time All-Star played for the Celtics from 2016-17 through 2018-19. Horford spent his five nine seasons with the Atlanta Hawks.
Brown, 21, averaged 8.6 points and 8.9 rebounds per game in 43 appearances this season for the Thunder.