Poised and confident in his Major League debut in a spot-start for the ailing Ervin Santana, O'Sullivan, 21, mowed down Giants hitters with the cool assurance of a veteran on Tuesday night en route to an 8-1 Interleague victory in front of 34,176 -- including a whole pack of O'Sullivans from the San Diego area -- at AT&T Park.
"I was pretty nervous out there," said O'Sullivan, who disguised it well on Irish Heritage Night at the ballpark. "I tried to throw strikes, let my defense work. We have a great defense behind us, and I wanted to keep my pitch count down, keep it in the zone."
The Angels' fifth win in a row -- all coming against the National League West -- was highlighted by another show of force, powered by Mike Napoli, with four hits, and Maicer Izturis and Bobby Abreu, with three hits apiece.
Homers by Napoli and Izturis along with four doubles gave the Angels 15 homers and 33 extra-base hits during this five-game offensive binge.
But the story on this occasion was the big kid from El Cajon, Calif., outside San Diego, who began the season at Double-A Arkansas and was promoted from Triple-A Salt Lake, where he was 5-2 in 10 starts.
O'Sullivan went seven innings, yielding one run on five hits and one walk, striking out five. Justin Speier and Jason Bulger finished the job, enabling the rookie to enjoy a night of celebration with his family members before heading back to Salt Lake.
O'Sullivan will trade places with right-handed reliever Rafael Rodriguez, who has been riding the Salt Lake shuttle for the Angels this season, compiling a 7.36 ERA in eight big league games as a middle reliever. He has a 0.68 ERA in eight games for the Bees.
Santana, experiencing discomfort in his right forearm, is expected to be back for a start Tuesday at home against the Rockies. But if he can't make it, O'Sullivan will be a phone call away.
"One thing about Sean, if you look at his Minor League career, is he wins games," Angels manager Mike Scioscia said. "He's not going to light up a radar gun, do anything that makes you say, `Wow.'
"He pitched a great ballgame. Early on maybe he was a little behind in the count, but he used his breaking ball well, pitched inside well and later on brought his changeup into the game."
Not until his wild pitch following Edgar Renteria's two-out single in the sixth did San Francisco put a runner in scoring position. The Giants ended his shutout bid in the seventh on Bengie Molina's single, Pablo Sandoval's double and a sacrifice fly by Nate Schierholtz.
O'Sullivan also delivered an infield hit in his second at-bat, accomplishing something it took Angels ace John Lackey seven years to achieve one night earlier with his first Major League hit in a regular-season game.
Like Lackey in Monday night's win, O'Sullivan was able to trot home on a home run, this one lashed by Izturis, his first of the season, into the left-field corner against starter and losing pitcher Jonathan Sanchez (2-7).
Napoli had unloaded a no-doubt, three-run shot an inning earlier, his ninth of the season.
Doubles by Izturis and Juan Rivera handed O'Sullivan a first-inning lead.
"In this game," O'Sullivan said, "you have to be an extreme competitor. My philosophy is when we score runs to put up a zero and keep the momentum on our side of the field."
The Giants went down quietly in the first and second innings before the Angels gave O'Sullivan serious breathing room with a four-run third inning.
Two errors on an Izturis grounder by first baseman Sandoval, who had committed another error an inning earlier, kick-started the surge.
Abreu delivered an RBI single and Rivera's second hit preceded Napoli's blast to left-center. With eight hits in his past 17 at-bats, Napoli has busted out of a 3-for-37 slumber.
Izturis' two-run shot in the fourth came after third baseman Rich Aurilia couldn't handle O'Sullivan's sharply hit grounder, with the ball arriving at first as a souvenir.
Having booked flights for his family when he found out he was getting the call, O'Sullivan was rewarding them with all sorts of firsts.
"It was absolutely everything I thought it would be," O'Sullivan said. "The lights, the people, I had family in the stands ... against a team I watched growing up."
Batting for O'Sullivan, Vladimir Guerrero had his second pinch-hit single in two nights in the eighth, scoring after singles by Izturis and Abreu.
"We are playing a hot team," said Giants manager Bruce Bochy, who commanded the Padres when they were O'Sullivan's hometown team growing up. "They have been swinging the bats well.
"We have got to pitch better -- and more than anything, you have got to play your best ball when you are playing good teams."
The Giants were coming off a weekend sweep of the A's when the Angels arrived, and they'll ask reigning NL Cy Young Award winner Tim Lincecum to put the brakes on an offense that has produced 17 runs and 31 hits in two games.
Lyle Spencer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.