In fact, many of O'Sullivan's best memories as a child revolve around going to Padres games with family, which is why it will be such a big thrill Friday (12:05 p.m. PT) when the right-hander gets a start for the team in its game against the Mariners at the Peoria Sports Complex.
"I was a Padres fan as soon as I knew what baseball was," said O'Sullivan, who is in big league camp as a non-roster invitee. "When I came into the offseason as a free agent, I knew if this opportunity presented itself, I was going to take it."
O'Sullivan, 25, is likely to throw two innings against the Mariners. He wore a Padres uniform for the first time on Monday for photo day, though this will feel significantly different.
He is a Padre, at last.
O'Sullivan, who graduated from Valhalla High in El Cajon, spent all of last season in Triple-A. He went a combined 14-7 with a 4.23 ERA in 31 games, including 19 starts, for Omaha and Las Vegas. He was a lot better with Las Vegas, going 9-3 with a 2.72 ERA in 14 starts.
"I didn't feel like I could be beat," O'Sullivan said of his time with Las Vegas.
But there have been times when he's gotten away from what serves him best -- pounding the strike zone. That was the case in stints with the Angels and Royals from 2009-11, when he posted a 10-14 record and a 6.13 ERA in 43 games (34 starts).
"My biggest thing is attacking the zone early," said O'Sullivan. "It's when I try to get too cute with my pitches that I start to go off the rails. When I attack the zone with ferocity, that's when I'm at my best."
Winning a spot in the rotation won't be easy. The Padres have no fewer than 10 candidates trying to win a job in the rotation. The good news for pitchers like O'Sullivan is there will be ample chances to do so, as the team will play a club-record 38 exhibition games before Opening Day on April 1.
Could O'Sullivan crack the rotation?
"The thing that stood out for me when I saw him this winter at Petco [was that] he looks in great shape," said Padres manager Bud Black. "[He's] really determined to make his mark. He's been with a couple different teams with some varied success, but there is a determined look in his eye, which is great."
O'Sullivan, who said his preference is to start, could offer the team a versatile arm moving forward should he make the Opening Day roster. If that happens, it will be a dream come true for the Rancho San Diego resident and his family.
"I'm happy with what I'm doing right now, where I'm at and knowing if I make the team, I can go home to my team," he said. "That opportunity would be priceless."