"He is the perfect fit for our organization and for our community and we look forward to a long and successful relationship," Thunder chairman Clayton Bennett said.
Donovan, who recently signed a one-year contract extension that would have paid him an average of $4 million through 2020, led the Gators to four Final Fours, seven Elite Eights and 14 NCAA tournament berths in 19 years.
He will be under pressure to produce a winner in Oklahoma City immediately. His roster will include 2013-14 NBA MVP
Facing numerous injuries to key players, the Thunder finished 45-37 and missed the playoffs this season.
"I knew that it would take a unique opportunity to leave the University of Florida, and that is clearly how I look at this situation," Donovan said.