HOUSTON -- Ryan Cook didn't start the season on the active roster as he got over this spring's shoulder tightness, but he's steadily moving up in the A's bullpen since he's returned.
He has not allowed a run in his last four appearances and has allowed just one in seven games over 6ï»¿2/3 innings for an ERA of 1.35.
As he gets stronger and more consistent in the strike zone -- he's allowed just two hits but has walked six -- he's being used later in games.
Manager Bob Melvin said he's not ready to have Cook join the closer by committee mix he has with Luke Gregerson, Sean Doolittle and now Jim Johnson, but he's impressed by the progress Cook has made.
"He's been good," Melvin said. "He came out a little late, but it's getting to where we can put him into more prominent roles."
But, Melvin said, "not in the ninth inning right now.''
When the A's faced Oberholtzer last week Moss was on the bench and the A's started two lefties, Josh Reddick and Daric Barton. Reddick and Barton started against Texas lefty Martin Perez on Wednesday, and they both went 0-for-3 in the 3-0 loss.
In the start against Oberholtzer, both Reddick and Barton had hits off the lefty in the sixth inning, and it was those hits as much as anything that ended the outing for Oberholtzer.
Melvin said the decision to go with Moss Thursday was based "on some variables" that he didn't want to get into. He did say the situation with Moss is different now than in 2013 with first baseman Nate Freiman, a right-hander with some power, no longer on the roster.
The manager did say that switch-hitting Alberto Callaspo, who was groomed to be the first baseman against lefties this spring, would be at first base soon, possibly on Saturday against Houston lefty Dallas Keuchel.
As the Oakland roster is currently constructed, Melvin will have to play at least one lefty against left-handed starters, and it may come down to a choice between Moss and Reddick, who has been warming up lately. Reddick has nine hits in his last 21 at-bats, though he's just 1-for-6 against lefties in that stretch.