Buckley will play Clayton, the sports agent who takes on NBA-bound Nathan as a client and friend, says the Hollywood Reporter.
Brian Austin Green was until recently eyed for this series regular role, but ultimately a deal could not be worked out.
Buckley's Tree Hill gig stems from a holding deal he had with Privileged producer Warner Bros. TV.
One Tree Hill is adding three new characters for Season 7, as the show moves on without longtime leads Hilarie Burton and Chad Michael Murray. The two characters remaining to be cast are Haley's sister, Quinn, and a supermodel who goes to work for Brooke.