He faced six batters, pitching to Bobby Kielty, J.J. Furmaniak and Chris Snelling twice each.
He struck all three of them out the first time through, then he got Kielty to pop up, Furmaniak to hit a lazy fly ball and Snelling to hit a roller to the right side.
"I felt great," Harden said. "Overall, everything went well. It was a step in the right direction toward getting back out there."
Harden threw nothing but fastballs and changeups and will work in his other pitches when he makes a rehab appearance with Triple-A Sacramento on Wednesday.
"We'll work everything else in next time out," he said.
Harden will pitch out of the bullpen with Oakland while he continues to build his pitch count. He will start with about 25-30 pitches and make his way up.
Geren said in Spring Training they usually work in 15 pitch increments, which means he would throw 40-45 the next time until he was ready to throw 75-80 and start a game.
"I haven't thought that far ahead," Harden said. "I'm just looking forward to getting out there soon."
And soon it may be, with the A's hoping to have Harden with them on Friday when they open up a series in New York with the Mets.
"If everything goes well, he'll be with us this Friday," Geren said.
Chavez returns, Komine called up: Eric Chavez, who sat out of the final two games of the St. Louis series with a strained muscles in his left rib cage, returned for Monday's game with the Reds.
Chavez was 5-for-13 with two home runs off Reds starter Kyle Lohse prior to Monday's game.
The A's also made a roster move on Monday, calling up right hander Shane Komine from Sacramento and sending down righty Ruddy Lugo.
Komine was 3-8 with a 5.31 ERA with the Rivercats this year, and had a 5.00 ERA without a decision in two appearances for the A's last season.
Lugo was claimed off waivers from Tampa Bay on Thursday and pitched twice. He had a scoreless appearance on Friday but then gave up seven earned runs in two-thirds of an inning on Saturday. He is 2-0 with a 13.14 ERA between the two teams this year.
Doolittle, three others inked:
The A's came to terms with four picks from the 2007 First-Year Player Draft, including compensation pick Sean Doolittle.
Doolittle was taken as a sandwich pick between the first and second round as the A's got compensation due to the Giants' signing of Barry Zito.
Doolittle earned first team All-Atlantic Coast Conference, hitting .301 with 14 doubles, seven home runs and 53 RBIs at the University of Virginia, and was also 8-3 with a 2.40 ERA on the mound. His ERA was second in the conference.
"He's a good kid," Geren said. "He's a line-drive guy, and he's happy to be here."
Doolittle was in Oakland before Monday's game and went through infield drills and took batting practice with the team.
"It happened so fast," Doolittle said about stepping into a big-league batting cage for the first time. "It's one of those things that you never think is gonna happen until it happens."
The A's also signed left-hander Lance Sewell, a seventh-round pick out of San Diego State, right-hander Justin Friend, a 13th-round pick out of Oklahoma State, and catcher Matt Smith, a 14th-round pick out of Texas Tech.
They have now signed 22 of the 53 players they drafted.
Milton Bradley, who made two rehab starts in Sacramento on Saturday and Sunday, was eligible to come off the disabled list before Monday's game but was not activated despite being back with the team. ... Injured outfielder Kielty should be going out on a rehab assignment soon. He will likely go Wednesday or Thursday to either be Sacramento or the club's extended Spring Training facility in Phoenix.
A's righty Chad Gaudin (6-1, 2.85 ERA) will take the ball opposite Reds righty Homer Bailey (1-0, 5.73) on Tuesday night. First pitch is set for 7:05 PT at McAfee Coliseum.