Colorado (21-10) needed a couple of wins in Las Vegas to boost its NCAA chances and got off to a good start by dominating Oregon State defensively most of the second half.
Xavier Johnson had 16 points and Andre Roberson 12 for the fifth-seeded Buffaloes, who moved on to face No. 4 seed Arizona in Thursday's quarterfinals.
Oregon State (14-18), the No. 12 seed, pulled off one of the biggest upsets in Pac-12 tournament history by knocking off top-seeded Washington last year and dimmed Colorado's NCAA hopes with a win in Boulder last weekend.
The Beavers fell short of repeating either performance by struggling against Colorado's defense, hitting 7 of 25 from 3-point range.
Roberto Nelson had 20 points, and Eric Moreland had 14 points and 13 rebounds for Oregon State, which went 3 for 17 from the arc in the second half.
Colorado had a 43-30 rebounding advantage, including 17 on the offensive glass that led to 18 points.
Colorado entered the Pac-12 tournament on the NCAA bubble after losing its season finale to Oregon State at home.
Playing without Roberson, who missed his second straight game with a viral infection, the Buffaloes struggled against the Beavers, losing 64-58 right after knocking off then-No. 19 Oregon.
Oregon State had lost five straight before that and hadn't won in Boulder in six tries, so it was one of those late-season losses that won't look good on Colorado's resume when Selection Sunday rolls around. It also meant the Buffs lost a chance at getting an opening-round bye in the Pac-12 tournament, dropping the defending champions to the fifth seed.
Coach Tad Boyle was so frustrated by the loss that he grabbed a microphone and apologized to the fans after the listless performance in the final home game for Colorado's seniors.
The good news was that the Buffs only had to wait four days for the rematch.
It was ugly early.
Colorado missed eight of its first 11 shots, Oregon State opened 1 for 6, and the teams needed more than five minutes to crack double figures in combined points.
Luckily for the teams and their fans, the action picked up a little.
Colorado got little production from Roberson, who had 2 points on 1 of 3 shooting and two fouls in seven minutes.
The Buffaloes made up for his quiet half with 10 offensive rebounds that led to 11 points and a 32-26 halftime lead after Dinwiddie hit a half-court shot at the buzzer.
Oregon State didn't score in the final 2:42 of the first half and missed its first six shots of the second. The Beavers kept misfiring, falling behind 53-42 after missing 16 of 20 shots, including all 10 of their 3-pointers.
Colorado answered every rally attempt by Oregon State after that, hitting 11 of 16 free throws in the final 1:53 to earn a chance at picking up another important win.