Okajima was 0-1 with a 2.16 ERA and .193 opponents batting average in 14 games with the River Cats. He walked two and struck out 18 in 16 2/3 innings.
"He's been throwing the ball well, against left-handers in particular," A's manager Bob Melvin said. "It seems like you always need a couple of lefties for a game. To add another left to the mix will be good for us."
The 37-year-old left-hander has not allowed a run over his last nine games, a span of 11 innings, and has stranded all four of his inherited runners over that stretch.
This marks Okajima's sixth season in the Major Leagues as he went 17-8 with six saves and a 3.11 ERA in 261 games with the Boston Red Sox from 2007-11.
"I want to be able to pitch like I did my first year in Boston," Okajima said through an intrepreter. "I was hoping to get back to the Majors and the A's gave me the opportunity."
Resop was 1-1 with a 6.00 ERA and .293 opponents batting average in 18 games with the A's. He did not allow an earned run over his first seven games this year, but has surrendered 12 runs over his last 11 2/3 innings for a 9.26 ERA.
"Resop fits in here," Melvin said. "He had a great spring. We'll have to see if he clears waivers and we can keep him. I have nothing bad to say about him."
Rick Eymer is a contributor to MLB.com This story was not subject to the approval of Major League Baseball or its clubs.