The Nationals need a reliever who can pitch a lot of innings, because the bullpen is taxed and they can't use Chad Cordero because of shoulder problems. King is considered a lefty specialist who can get only lefties out, and it was rare when he pitched a full inning. O'Connor will be the long man out of the bullpen.
"With our bullpen right now, we can't afford to have a lefty specialist like Ray to pitch to one or two guys," manager Manny Acta said. "We need a guy who can give us more than one inning, could pitch to lefties and righties and bridge the gap -- like O'Connor."
King, 34, last pitched on Tuesday against the Mets, giving up two runs in two-thirds of an inning. He pitched in 12 games and gave up four runs in 6 1/3 innings. King has 72 hours to accept the assignment. In the meantime, he is hoping that the Nationals can trade him or give him his release.
"After 14, 15 years, they come in and say that you don't have a job, it's very surprising," King said. "When things go bad, we start looking for other avenues. I'm going to take today and the next day to talk with family, talk to my agent and see what's out there. I don't want to make a decision or say something that could hinder me from doing other things.
"I'm not upset about anything. If I don't go to another team and I have to go home, I've had a great career. I had a chance to pitch for great teams, bad teams and in the World Series. ... I don't want to go to Triple-A, but I'm just going to take the next couple of days and see what's out there."
O'Connor, 27, joins the Nationals after going 2-0 with a 1.96 ERA in four starts with the Clippers. He received word of his promotion at 2:00 p.m. ET on Thursday and arrived at Nationals Park at 5:20 p.m.
A product of George Washington University, O'Connor is arguably the biggest comeback story in Nationals history. A year ago at this time, he wasn't on the team's radar screen because of an elbow injury. When he came back to pitch for Double-A Harrisburg, O'Connor was hit hard and had a 7.07 ERA in 15 starts.
In Spring Training, O'Connor came back and had a respectable 3.77 ERA.
"Last year, I came back too soon," O'Connor said. "When I came to Spring Training, I felt everything was back to normal. It feels good to get back, knowing what I went through last year."
O'Connor last pitched in the big leagues in 2006, when in 21 games, he was 3-8 with a 4.80 ERA. The Nationals still consider him a fifth starter in the future. For now, O'Connor will accept any role that is given to him.
"I've relieved in the past," O'Connor said. "I feel comfortable doing it. I have no problem with either role. I know I can do both."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.