McDonald made a 3-pointer just before the first quarter buzzer and the Wildcats trailed 19-18.
Oregon State went on a 13-2 run to close out the second quarter, with Pivec and Maddie Washington scoring all the squad’s points, for a 37-28 lead at the break.
The Beavers took control of the game with a 10-2 surge to open the third quarter, and led 47-30 after a layup by Slocum at the 6:22 mark.
Arizona: McDonald, who sat out last season after transferring from Washington, came into the game averaging 25.6 points, tops in the Pac-12 and third in the nation. . The Wildcats haven’t been ranked since the 2004-05 season, but received poll votes this week.
Oregon State: The Beavers have won their last 10 games against Arizona. . Oregon State leads the nation in 3-point shooting percentage at 49.6 percent.