Trailing 30-28 at the half, allowing UCLA to shoot 57.1 percent from the field, the Trojans knew what they had to do.
"We just got to pick it up," senior guard Marcus Simmons said. "We just came together as a team before KO came in the locker room and said we got to pick it up and protect the home court."
USC responded with a dominant second half, holding the Bruins to 25.9 percent shooting and pulling away for a 63-52 win Sunday night, their fourth consecutive victory in the cross-town rivalry.
Buoyed by a sold-out Galen Center, including a student section that was full nearly an hour before tip off, the Trojans (10-6, 2-1 Pac-10) turned in the kind of defensive effort that has been O'Neill's trademark.
"It definitely helped pick us up," forward Alex Stepheson said of the crowd of 10,258. "It was the first time in a while the crowd was really huge like that, especially the student section. It really was the sixth man for us tonight and gave us that energy boost."
Forward Reeves Nelson, who scored 12 of UCLA's 30 points in the first half, had just two after the break.
Nelson and mammoth center Joshua Smith "set the tone," as O'Neill put it, in the first half with a series of dunks and lay-ups.
Smith, generously listed at 305 pounds, played just 22 minutes because of foul trouble but was a force when he was in the game.
"That's the biggest dude I've seen, especially when he's setting screens," Simmons said.
"You can't say that enough," Stepheson said. "That's a big man. You can't push him out the way. Whatever he wants to do, pretty much you can't stop him from doing it."
The Bruins' other leading scorer, forward Tyler Honeycutt, was locked down in typical Simmons fashion, managing just 10 points on 3-of-10 shooting from the field while committing 7 turnovers.
"For a night, we were able to stop them," O'Neill said.
As they did on defense, so too were the Trojans able to flip the switch on offense.
Stepheson had 13 points and 16 rebounds in his first game playing with a healthy left hand. The senior broke it in the season opener and had been alternating wearing between a brace on game day and cast the rest of the time.
"The first half was a little bit difficult," Stepheson said. "I had to mentally get used to putting it out there again without fear of it getting hit."
Nikola Vucevic added a game-high 20 points, while guard Jio Fontan chipped in with 10.
"When you beat UCLA, you've beaten tradition," said O'Neill, who improved to 3-0 against the Bruins (9-6, 1-2 Pac-10).