Fisher, who won five NBA titles as a member of the Los Angeles Lakers, was traded by Los Angeles to Houston last week, but never played for the Rockets, instead opting to buy out his contract so he could join a championship contender for the stretch run.
Oklahoma City General Manager Sam Presti said the 16-year veteran will give the Western Conference-leading Thunder added depth at the guard position.
"He is someone that will enhance our roster's current and future development on and off the floor through his knowledge, professionalism and approach to the game," Presti said.
Fisher was averaging 5.9 points, 3.3 assists and 2.1 rebounds per game this season before being dealt by the Lakers.
The 37-year-old sports career averages of 8.7 points, 3.1 assists, 23.1 rebounds and 1.1 steals per contest.