"A dream come true is the only way I can describe it," he said prior to Thursday's game, already one of the first players dressed in uniform for pregame warmups. "I always dreamt as a little kid about putting this uniform on, so it's a special day."
When O'Sullivan found out he would make the start Friday, it surprised him. He knew the spot in the rotation was open, and he'd pitched well at Triple-A Tucson with a 7-4 record and 3.88 ERA in 18 starts. He pitched well in Spring Training with a 1.35 ERA in 13 1/3 innings.
But he thought one of the other pitchers at Tucson that the Padres have already used in the Majors this season would get the call. Besides, he wasn't even on the 40-man roster.
"I wasn't expecting it, so it's kind of a great way to be caught off guard," he said.
The Padres also will have to make a corresponding move before Friday's game to add O'Sullivan to the roster.
O'Sullivan only reserved tickets for his family Thursday, but he has already heard from other friends that they're buying tickets. He admitted with all the excitement of pitching in his hometown, it will certainly be hard not to put any added pressure on himself.
"It's hard not to want to do so well," O'Sullivan said. "I'll try to put that as far back in as I can and go out there and play baseball."
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter. Jamal Collier is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.