The lesson took, as they won six of their next eight, including a dominant showing over nationally ranked Texas and narrow losses to Kansas and Washington.
But USC regressed last weekend at the Oregon schools, so O'Neill was again left to stress the need to play outstanding defense all the time.
"We're not going to beat anybody offensively very often. We proved it the last three games," O'Neill said.
"If you forget who you are, you're not going to win."
The lesson took again, as the Trojans bludgeoned hapless Stanford, 65-42, Thursday night at Galen Center.
"It was a must-win for us," senior guard Donte Smith said. "We had a sense of urgency to come home and get this first one."
Buoyed by a last-minute scoring burst against walk-ons, the Cardinal (10-7, 3-3 Pac-10) managed to shoot 22.2 percent from the field, still the lowest mark in school history in the shot clock era.
Leading scorer Jeremy Green was held to five points, one-third of his average.
Smith said that's the formula USC (11-8, 3-3 Pac-10) needs to have success.
"We just got to stay focused on the defensive end," Smith said. "Going through our season, if we hold people under 30 percent, we win the games. They shoot over 50, we lose. That's all it is. When we play well on defense, it makes things easier on offense."
That held true, with the Trojans shooting a blistering 55.6 percent, their second-best performance of the season.
Forward Nikola Vucevic led all scorers with 20 points, while Alex Stepheson recorded his seventh double-double of the season with 10 points and 11 rebounds.
Vucevic believes the lesson has stuck for good, with the recent lull the by-product of an inexperienced roster.
"It's a maturity thing," he said. "We've got a really young team, young guys and guys that haven't played a lot. As the year goes on, we'll get better at that."
Freshman Garrett Jackson added six points in 11 minutes, solidifying his spot in the rotation following the announcement earlier in the week by freshman guard Bryce Jones that he would transfer.
While O'Neill thought the team showed no reaction to Jones' departure, some players disagreed.
"It felt different without Bryce there," Smith said. It was weird, but at the end of the day we had to man up and get the job done. We're going to miss Bryce. He's a good guy."