O'Flaherty has 14 consecutive scoreless appearances to whittle his ERA to 1.88 and opponents' average to .188, including .150 (3-for-20) by lefty batters.

"I'm pretty happy with the way everything's going --- couldn't go much better," said O'Flaherty, 24, who was claimed off waivers from Seattle in November.

After posting a 4.47 ERA in 56 appearances as a Mariners rookie in 2007, he missed most of last season with a severe back strain.

His 5.91 ERA in 78 major league appearances before this season didn't do much to boost confidence of Braves fans when team officials mentioned O'Flaherty as a replacement for Ohman.

But after four hits and three runs in 2 1/3 innings during his first four Atlanta appearances, O'Flaherty has reeled off 12 scoreless innings of five-hit ball with no walks and seven strikeouts.

He has stranded nine of 10 inherited runners, tied for the fifth-best percentage among National League relievers through Saturday.

"Coming into the season, I was focused on being healthy and pitching the whole year," said O'Flaherty, who has used a stretching and core-muscle strengthening program to avoid further back problems.

"I'm just throwing strikes, that's the big [key]. It's fun the more you're out there pitching, because you stay fresh. When you throw every day, almost, either in the game or in the bullpen, it's pretty easy to repeat your delivery."

Medlen's debut delayed to Thursday

Kris Medlen will have to wait a little longer for his big-league debut. Because of a change in plans after Sunday's rainout, the 23-year-old right-hander is set to start Thursday against Colorado, instead of Tuesday.

Braves veteran Derek Lowe had his rained-out start Sunday moved to today, and Jair Jurrjens' start was bumped from today to Tuesday.

Javier Vazquez was the only starter not affected. He's still scheduled to start Wednesday against the Rockies because the Braves didn't want him to have extra rest for the second start in a row.

Manager Bobby Cox said Medlen would probably pitch a side session today or Tuesday before he starts the finale of the four-game series with the Rockies.

Medlen pitched in relief Saturday for Class AAA Gwinnett, which was to have served as a between-starts bullpen session. That was reduced to six pitches when the game in Pennsylvania was rained out after Medlen recorded one out.

Cox said he wasn't concerned about the rookie staying sharp.

"He'll be fine," Cox said. "He's not going to forget how to pitch."

Medlen has been sensational for Gwinnett, going 5-0 with a 0.96 ERA and 44 strikeouts.