O'Day, who entered in the ninth inning against the Indians, said he was oddly enough more calm than usual in his return.
"I think it's just being gone, being away from the guys and the competition, makes you appreciate what you get to do and to be in the moment and get to pitch in a big spot," O'Day said. "There are guys standing out there in 90-something degrees, and to be able to keep that going and perform in that arena, I think it really helped me a lot."
O'Day, who was placed on the DL June 3 with a right hamstring strain, threw a scoreless inning at Bowie on Friday in his lone rehab appearance. On Saturday, he said he felt no pain during the outing and was ready to return to Baltimore.
It was a welcome return for the fans at Camden Yards as well, who chanted O'Day's name while he was on the mound.
"That was cool. I didn't notice it until [there were] two outs. It's pretty neat," O'Day said.
"I love it cause if they are doing it, that means I'm pitching well. It was a lot of fun. I don't know if I could have written it any better. The team, the walk-off homer, Vance [Worley] -- our starting pitching has been awesome the last few days here and they make it a lot easier on the bullpen -- so you can't talk about the game without talking about Vance."
O'Day, who earned the win, is now 3-1 with a 3.00 ERA on the year. He's struck out 28 batters in 21 innings and is a major boost to an O's 'pen that entered Sunday with the fourth-best ERA in the majors (3.10).
To clear a roster spot, the Orioles optioned left-hander Donnie Hart to Double-A Bowie.
Hart, who was recalled July 15 for his first Major League stint, pitched in three games with the Orioles. He didn't allow any runs in 2 2/3 innings.
Ryan Baillargeon is a reporter for MLB.com based in Baltimore.