Eric Wise added 12 points and 11 rebounds, J.T. Terrell and Byron Wesley each had 10 points for the USC (9-13, 4-5 Pac-12), which held the Bruins to 38 percent shooting while snapping a two-game losing streak.
The Trojans improved to 2-3 under interim coach Bob Cantu.
UCLA's Shabazz Muhammad scored 22 points but missed two 3-pointers in the final 1:14 of overtime. The Bruins were 2 of 19 from 3-point range.
Travis Wear, who returned after missing one game because of a concussion, added 16 points and Jordan Adams had 13 for the Bruins.
UCLA (16-6, 6-3) has dropped three of their last four and are in danger of slipping out of second place in the league after losing their only game this week.
The Bruins trailed by 15 points early in the second half and played from behind until Adams hit a jumper with 30 seconds left in regulation to tie the game at 65 and force overtime.
In the extra session, Adams' layup gave UCLA a 69-67 lead, its first since 6:49 of the first half. Fontan hit a 3-pointer before Muhammad's basket put UCLA back in front 71-70. Wesley answered for USC with a jumper that gave the Trojans a 72-71 lead. Travis Wear fouled Ari Stewart, who made both free throws for a 74-71 lead with 11 seconds left. Muhammad missed another 3-pointer and then Adams committed his first foul with 4 seconds left, sending Fontan to the line where he sealed the victory.
Playing in a less than full Pauley Pavilion, the Bruins moved out to a 21-15 lead to open the first of two regular-season meetings between the rivals. USC dominated the final 7:16 of the half with a 23-9 run that gave the Trojans a 38-30 halftime lead. Fuller scored 11 points in the spurt.
USC picked up where it left off to start the second half, going on a 14-7 run and getting four points each from Dewayne Dedmon and Wise to extend their lead to 52-37.
The Bruins started rallying with a 3-pointer by Muhammad and a more active defense. His basket keyed a 19-9 run during which Fontan picked up his fourth foul for the Trojans after scoring five straight points in the spurt that closed UCLA to 61-56. Muhammad had six points and Wear and Larry Drew II had four each.
Muhammad stole the ball from Fontan and scored to draw UCLA within two. After Fontan scored, Drew II made a layup that again brought the Bruins to 65-63. After passing the ball around, they tied the game at 65 on Adams' short jumper.
Wise got called for his fourth foul going to the basket with 3 seconds left. After a timeout, Drew II dribbled up the court and launched a 3-pointer that hit the front of the rim as time expired, forcing overtime.
The Bruins wore alternate all-blue uniforms that featured solid color blocking of the team logos and a shimmery finish.
Reggie Miller had his No. 31 jersey retired at halftime. He starred for the Bruins from 1984-87, helping them win the NIT title as a sophomore. It's the second No. 31 jersey in the rafters at Pauley Pavilion. Ed O'Bannon wore the same number as the star of UCLA's 1995 national championship team.
"Let's bring back those short shorts I was wearing in that video," Miller told the crowd.
Later, he said, "I never thought something like this would happen. It's humbling."
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