Trey Kell made a steal and a transition layup to give San Diego State a 55-53 lead with 1:14 left. Then Skylar Spencer blocked Cody Doolin's layup attempt and O'Brien drove the lane for a four-point lead with 18 seconds left.
San Diego State (23-7, 13-4) is tied with Boise State (23-7, 13-4) for first place, but has lost to the Broncos twice this season.
Winston Shepard added 10 points and eight rebounds for San Diego State. Spencer had three blocked shots to give him a school record 216 in his three-year career. Leonard Allen (1982-85) held the previous record of 214.
Christian Wood had his 17th double-double of the season with 10 points and 16 rebounds for UNLV (16-14, 7-10).