Shut down the opponent's best guard.
"If you give him five things to do, he might not do them very well, but if you give him one, he does it exceptionally well," O'Neill said.
Simmons did just that Friday afternoon, limiting Washington State swingman Klay Thompson to 17 points as the Trojans held off the Cougars, 60-56, for their first Pac-10 win at Galen Center.
Thompson, the preseason pick for conference player of the year, needed 18 shots to do so. Most of his buckets came off broken plays, which allowed the junior to connect on four three-pointers.
Other than those few instances, Simmons kept Thompson under wraps, with the Pac-10's leading scorer going 6-18 from the field.
And unlike other teams that might use multiple defenders against a player of Thompson's caliber, O'Neill put Simmons in "zero cover," straight man-to-man coverage.
"He doesn't do anything except guard that (one) guy," O'Neill said. "Marcus has become as good a defensive stopper in college as you could want to have."
"I think I can guard anybody," said Simmons, who went from defending Washington point guard Isaiah Thomas on Wednesday night to the 6-foot-6 Thompson.
But Simmons' presence comes at a price, offensive limitations to the point that teams will only make a token effort to guard him. Still, the senior from Alexandria, La. managed 7 points on 2 of 5 shooting, including a three early.
"When I hit the three, I laughed to myself," Simmons said. "I mean my shot has been pretty off these last couple games."
Against the tough WSU zone, however, USC (9-6, 1-1 Pac-10) needed someone else to hit outside shots, a deficiency that cost the team a chance to beat the Huskies.
Fortunately, Donte Smith stepped into the void with 14 points, finding the stroke that made the Trojans so formidable in high-profile non-conference games against Texas, Kansas and Tennessee.
The senior was 3 of 4 from the field in the second half, including back-to-back threes to push the lead to 11.
"Donte's become kind of a clutch guy for us," O'Neill said. "He's made some big shots for us and did again today."
The Cougars (10-4, 0-2 Pac-10) rallied within two points with 1:19 left to play on an 8-0 run, but Smith hit two late free throws to ice the game.
Said Smith: "I love pressure. KO called a time out and everybody was like, ‘why did he do that?' I just knew I was going to knock them down.
"Me, as a captain, I wasn't going to let us lose tonight."