Devon Collier added 14 points for the Beavers, who improved to 11-2 at Gill Coliseum on the season. Oregon State has three of its five remaining games at home.
DaVonte Lacy scored 18 points for Washington State and D.J. Shelton had 10 points and 11 rebounds.
After the Beavers made the first basket of the game, Washington State went on an 8-0 run.
The Cougars promptly went cold, however, and Oregon State outscored Washington State 25-6 over the next 12 minutes. The Beavers took a 27-16 lead with 4:34 remaining.
Washington State didn't score for more than four minutes during that stretch. But Oregon State followed up with its own scoreless 4-minute span, and went to the break up 30-26.
After the opening minutes of the second half, the Cougars couldn't get closer than five points.
Oregon State beat Washington State 66-55 in the teams' first meeting this season on Jan. 22 in Pullman.
The Beavers six Pac-12 wins are more than or equal to its total conference wins in 15 of the last 20 seasons.