Brock Motum added 20 points and 10 rebounds for the Cougars (11-9, 2-5 Pac-12), who won their first road game in five tries this season.
Ahmad Starks had 17 points and eight assists for Oregon State (11-9, 1-6). Starks was wide of the basket on a potential game-tying 3-pointer from the top of the arc in the closing seconds. That followed Washington State's DaVonte Lacy's two missed free throws and Starks' layup with 7.9 seconds left to cut the Cougars' advantage to one.
OSU's Royce Woolridge then made two foul shots, setting up Starks' final attempt.
Ladd's lay-in gave Washington State its first lead of the second half at 37-36 after the Cougars trailed by four at halftime.