O'Connor, viewed by many as the Nationals' top pitching prospect in 2005, was often hit hard during his brief time in the big leagues, going 4-9 with a 5.45 ERA in 26 games. This year, O'Connor was 2-6 with a 4.65 ERA for Syracuse and Double-A Harrisburg before the trade.
"We needed roster space, and Mike was the guy we chose to move -- and there was interest with another club," acting general manager Mike Rizzo said. "We are looking out for him. We didn't want to send him back to Double-A. We felt this was the best course of action -- not only for us, but for him, too."
The Nationals have not figured out who would take O'Connor's place on the roster. They could keep the spot open and wait for left-hander Matt Chico to recover from Tommy John elbow reconstruction surgery, which has cost him much of the past two seasons. Chico is now healthy and pitching for Class A Hagerstown. In two games, he has given up two runs in six innings.
In other news, reliever Drew Storen, the Nationals' 10th overall pick in the 2009 First-Year Player Draft, is expected to be activated on the Hagerstown roster on Wednesday.
Storen hasn't pitched in a game in three weeks. Rizzo said the organization will get Storen into a throwing program so he can build up his arm strength.
"We are going to be cautious with him and allow him to adapt to the professional game," Rizzo said. "We want to extend him [in the future] so he can feel the rigors of being a professional pitcher."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.