The Celtics announced the move Thursday evening.
Poirier appeared in 22 games with Boston last season, averaging 1.9 points and 2.0 rebounds in 5.9 minutes.
The move comes after Celtics center Enes Kanter reportedly picked up his player option for the 2020-21 season earlier Thursday, per ESPN's Tim Bontemps. Boston also added another forward in Wednesday's draft, selecting Vanderbilt's Aaron Nesmith No. 14 overall.
Oklahoma City, meanwhile, added to its cache of draft picks with its recent trades of Chris Paul and Dennis Schroder/Danny Green and could afford to part with a conditional second-rounder for more size up front.
Whether that means Poirier will get meaningful minutes with the Thunder is unclear.
The big man out of France spent four games in the G League last year, where he proved more than competent, averaging 16.3 points and 9.0 rebounds.
He'll have an opportunity to try to replicate that success in the NBA with OKC.