O'Sullivan, making his first start this season for the Angels on July 21, defeated the Yankees with six strong innings of two-hit pitching over which he gave up two runs.
He'll be back on the Yankee Stadium mound Sunday, this time wearing a Royals uniform. Kansas City acquired the right-hander in a multiplayer trade on Thursday.
"The clubbies keep making jokes that I keep getting traded to the next team that comes in for a series," O'Sullivan said of the clubhouse crew that was busy at work in the cool visiting quarters at Yankee Stadium on Saturday morning.
What O'Sullivan will be doing Sunday is something of a rarity.
The last pitcher to face the same club for different teams in successive starts was the late Cory Lidle, who defeated the Rockies in Colorado for Cincinnati on Aug. 8, 2004, then wore a Phillies cap in a loss to the Rockies in Philadelphia on Aug. 12. The last pitcher to win twice in this situation was Andy Ashby, who defeated Baltimore as a member of the Phillies on July 8, 2000, and then as a Brave on July 14.
"The decision was more that it's his regular day," Royals manager Ned Yost said. "If his regular day was on Monday, we wouldn't have pitched him on Sunday. More than anything, it was his regular day to pitch."
Yost said he expects O'Sullivan to make some adjustments this time around and the manager figures the Yankees might take a slightly different approach than they did Tuesday.
"I imagine it's pretty tough, but we'll find out on Sunday," Yost said. "He had a pretty good outing against them. It is what it is. He just goes out and hopefully repeats his performances."
O'Sullivan didn't seem too worried about the task ahead.
"Just go out there and put yourself against guys who have been doing it for a long time," said O'Sullivan, a third-round Draft choice in 2005 who made 10 starts for the Angels last year. "Future Hall of Famers. My last start here was my first start here, so I had some pretty good butterflies in the first inning. But after that, I took a deep breath. It's just baseball."