O'Sullivan, who was scheduled to pitch Wednesday for Triple-A Tucson, is 7-4 with a 3.88 ERA in 18 starts in the hitter-friendly Pacific Coast League.
"Sean has pitched very well … it's much deserving," said Padres manager Bud Black.
The Padres opted to pick O'Sullivan over Tim Stauffer, as Black would prefer to keep him accustomed to working long relief or even short relief on occasions. Sullivan has been starting all season, and he has better stamina and would conceivably be able to work deeper into a game.
"With Stauff, he's been away from making starts for a while," Black said. "The pitch-count issue won't be a problem with Sean. I think this [O'Sullivan] works better for starting a game."
O'Sullivan, a San Diego native who attended Valhalla High and Grossmont College, has a 6.31 ERA in 43 big league games with the Angels (2009-10) and Royals (2010-11).
He signed a Minor League contract before the season and pitched well in Spring Training, posting a 1.73 ERA in 13 1/3 innings.
"He was one of the guys who it came down to making a tough decision at the end," Black said of the way O'Sullivan pitched in Spring Training.
He's not a bad hitter, either. O'Sullivan was 12-for-31 with a grand slam and 12 RBIs for Tucson. He had even been used as a pinch-hitter on five occasions.