BALTIMORE -- A costly walk, a stunning lapse in judgment and a foul bunt provided Eric Byrnes with the opportunity to break up a pitcher's duel.
Mark Kotsay had two hits and scored a run for the A's, who rebounded from an opening-day loss to take two of three from Baltimore.
A steady rain drenched the field from the outset and limited attendance to 16,395 -- the smallest crowd in the 14-year history of Camden Yards.
With the score 1-all, Marco Scutaro led off the Oakland eighth by drawing a walk from Steve Kline (0-1). Kotsay then bunted, and Kline's throw to first was intercepted by first baseman Jay Gibbons before it could reach second baseman Brian Roberts, who was covering the bag.
"It was my fault," Gibbons said. "I just assumed he was throwing it to me. Simple as that."
Despite Gibbons' mental error, the play was ruled a hit. Byrnes followed by fouling off a bunt attempt. That led Kline to believe Byrnes would try to bunt again, so he served up a fastball.
"If I throw a sinker down and away, he grounds out," Kline said. "He jumped me."
Byrnes hit the pitch over the center-field wall, and thrust his fist in the air to celebrate his first homer of the season.
"I was hoping he was going to throw another fastball and make me get the bunt down," Byrnes said. "Luckily, he did and I was able to hit it."
Byrnes came in with exactly one successful sacrifice bunt in 384 big-league games, and Kline had gone 34 consecutive games without allowing a homer.
Kline lamented throwing the fastball, but was almost as disappointed with the walk to Scutaro.
"You can't walk the leadoff hitter," the left-hander said. "That's where it started right there."
It continued with Gibbons' miscue.
"Never seen that play before," Orioles manager Lee Mazzilli said.
After the homer, Eric Chavez doubled and scored on a single by Erubiel Durazo for a 5-1 lead.
Ricardo Rincon (1-0) picked up the win with a perfect seventh, and Kiko Calero finished with two scoreless innings.
Roberts led off the Orioles' first inning with a home run, but Baltimore managed only four hits. After scoring four runs on opening day, the Orioles scored one run on seven hits in the final two games of the series.
"This club is going to hit. We just didn't do it the last couple of days," Mazzilli said.
The poor weather didn't cause a stoppage of play, and it certainly had no effect on the starting pitchers, both of whom were exceptional after yielding first-inning runs.
Danny Haren, who came to the A's in the trade that sent Mark Mulder to St. Louis, allowed one run and three hits in six innings in his Oakland debut. He struck out six, walked three and hit a batter.
"He gave us a chance to win," A's manager Ken Macha said, "but we didn't score until later."
Baltimore's Erik Bedard gave up one run and three hits in seven innings. The left-hander struck out five, walked one and hit a batter.
Oakland went up 1-0 in the first when Byrnes singled, moved up on a balk and scored on a single by Jason Kendall. Roberts tied it in the Orioles' half with his second career leadoff homer.
In the third, Haren plunked Sammy Sosa on the arm with a pitch that appeared headed toward his head. Sosa took a step toward the mound and yelled at Haren before being restrained by plate umpire Laz Diaz.
Bedard then hit Chavez with a pitch leading off the fourth, prompting Diaz to issue a warning to both benches.
It was 1-1 in the sixth when Sosa hit a one-out drive to deep left-center. He took a hop out of the batter's box in anticipation of seeing his 575th career homer, but the ball hit the warning track before bouncing into the stands for a ground-rule double.