Colorado's Sabatino Chen banked in a 3-pointer at the end of regulation but officials waved it off after reviewing the video, saying the ball was on his fingertips as the backboard lights went on to show time had expired.
Arizona (13-0, 1-0) outscored the Buffaloes 10-2 over the final 1:35 of regulation to force the overtime. The Wildcats were down by 17 points in the first half and 16 with 12:40 to go in regulation.
It was the third close win for the Wildcats, who beat Florida 65-64 and San Diego State 68-67.
Askia Booker led Colorado (10-3, 0-1) with 18 points.