He would have been an unrestricted free agent this summer and theoretically could have chosen to go to any team he wanted (including Boston). From the Thunder Rumblings blog:

The Oklahoma City Thunder has agreed to terms on a contract extension for Kendrick Perkins, the 6-foot-10 center acquired via trade with Boston last Thursday, The Oklahoman has learned.

Terms of the deal were not immediately known, but Perkins is eligible for a four-year extension. Perkins earlier this season had turned down a four-year offer worth approximately $22 million from the Celtics.

With the two sides agreeing to a deal now, the risk of Perkins leaving the Thunder as an unrestricted free agent this summer has been removed.

Perkins said in his introductory press conference Saturday that he wanted to be in Oklahoma City for “the long haul.”

“I hope I can be here,” Perkins said. “I (want to) be here. I couldn’t find a better situation for myself. I really want to be here. It’s just a great organization.”

Great move by the Thunder.  They acquired a young talent, signed him to a nice extension, didn’t give up anything, and simultaneously filled the biggest hole on their roster.  This people, is how you build an NBA franchise.

[Thudner Rumblings]