Angus Brandt scored 16 points to lead Oregon State while Devon Collier added 14 points for the Beavers (14-11, 6-7).
Brandt scored 12 points during the first 20 minutes, when the teams combined to record 16 turnovers.
Oregon State stretched its advantage to 43-33 on Collier's three-point play with 14:46 remaining. Washington State twice cut the deficit to five but could get no closer, and Collier scored consecutive baskets to seal it at 64-54 with 1:39 left.
Oregon State outscored Washington State 25-6 over a 12 minute stretch in the first half, taking a 27-16 lead with 4:34 remaining. But they lost most of that and it was 30-26 at the half.
But the Beavs pulled away again in the second half and the Cougars couldn't get close.
Oregon State came into the game led by Nelson at 21.3 points per game. He only had 12 points on 1-for-9 shooting against WSU, but added seven assists for the Beavers.
The Pac-12 Networks' Rich Waltz noticed Nelson was shaking out his hand much of the first half, wondering if he may have injured his wrist. There was no word from OSU head man Craig Robinson after the game, apart from saying Nelson was "beside himself" for the off night shooting-wise.