Arizona's T.J. McConnell missed an off-balance shot with 1 second left, and Oregon State fans rushed the court at Gill Coliseum.
Morris-Walker had 12 points and eight rebounds, and Gary Payton II had 10 points and nine rebounds for the Beavers (11-4, 2-1 Pac-12).
McConnell had 13 points and six assists, and Rondae Hollis-Jefferson scored 14 points for the Wildcats (14-2, 2-1), the third top-ranked team to lose Sunday. No. 2 Duke lost to N.C. State and fourth-ranked Wisconsin was defeated by Rutgers.
A gritty, defensive matchup, there were 20 lead changes and 15 ties. Neither team held more than a four-point lead.
The Beavers took a 56-52 lead with 1:33 left after Malcolm Duvivier was fouled on layin and made the free throw for a three-point play. Gabe York responded for the Wildcats with a three-pointer with 1:07 remaining.
Duvivier missed another layin, and Oregon State fouled Hollis-Jefferson on the rebound. He made one of two to tie it at 56-all.
Payton came into the game leading Oregon State with 12.4 points, 8.1 rebounds and 2.9 steals per game. The junior college transfer is the son of former Beavers All-American and NBA star Gary Payton.
Freshman Stanley Johnson came into the game leading Arizona with 14.4 points and 6.9 rebounds per game. He had scored in double figures in the previous 14 games, but had seven points Sunday.