Oregon State took the lead late in the first half and never trailed again on Saturday to stun Arizona 63-55, the Beavers' first victory in Tucson in 27 years.
It was one of the biggest wins yet for second-year Oregon State coach Craig Robinson, brother of first lady Michelle Obama.
The loss knocked the Wildcats (13-12, 7-6 Pac-10) out of a second-place tie with Arizona State in the Pac-10.
Roeland Schaftenaar led Oregon State (11-13, 5-7) with 14 points, while Calvin Haynes added 13 and Seth Tarver 11.
Jamelle Horne led Arizona with 15 pounds and 14 rebounds. Derrick Williams added 13 points and Kevin Parrom 12. The Wildcats' Nic Wise made 1 of 11 shots, 1 of 8 3-pointers, and managed five points.
The last Oregon State victory at Arizona came on Jan. 27, 1983, one season before Lute Olson became the Wildcats' coach.