BALTIMORE -- Orioles rookie Brian Matusz isn't one to point fingers.

"The balls found the holes," Matusz said of the three earned runs charged to his name on Tuesday night.

And technically that's all Oakland's seven hits -- six of which were singles -- were. But with a crowd of 11,987 at Camden Yards watching in dismay, the Orioles' 4-2 series-opening loss to the A's was made possible due to sloppy defense and an offense that has consistently been unable to back their young lefty starter. The loss, which snaps the O's season-high four-game win streak, handcuffed Matusz with a 2-9 record that is anything but deserved.

"It's frustrating," said Matusz, who tossed his ninth quality start but remains winless since his third start on April 18. "I'm frustrated now, but I'm not going to carry it over to tomorrow. I'll prepare for my next start and let it go. Because if I start worrying about that it's not going to make things any easier."

Things were hardly easy for Matusz on Tuesday night. Opening the last leg of a nine-game homestand, Matusz battled a notoriously-patient A's club, despite some shaky fielding behind him.

After allowing a pair of singles to open the third, Matusz got two quick outs and looked poised to keep the A's scoreless when Miguel Tejada made a great backhanded stop on Kurt Suzuki's ball. But Tejada's throw skipped slowly toward first, allowing Suzuki to reach on an infield single and Cliff Pennington to score.

Similarly, Ty Wigginton fumbled a would-be inning-ending double play ball in the fifth and was only able to record one out, leaving room for Coco Crisp to safely cross the plate. Matusz pitched around Tejada's subsequent error at third to get out of the inning and hold the A's to a two-run lead.

"If you just look at him pitch, you wouldn't think he's that young," interim manager Juan Samuel said of the 23-year-old Matusz. "He's got an idea. He knows what he wants to do on the mound, so when he needs to make good pitches, he's been able to make good pitches and get out of potentially big innings for the other club."

Matusz exited with one out in the sixth after allowing a seventh-inning single to Daric Barton, and was charged with his third run when Jason Berken surrendered a walk and an RBI single to Suzuki. The showers of applause from the Orioles' faithful as he walked off said it all: Matusz -- who allowed three earned runs or fewer for the fifth time in six starts -- had done his job.

"He's been doing that for us all year," Samuel said.

And the O's offense continued to do what it has done seemingly all season: keep the opposing starter in the game.

With originally scheduled starter Dallas Braden scratched with left elbow stiffness, the A's summoned right-handed Vin Mazzaro to make a last-minute spot start just hours before first pitch. Mazzaro, who had made just five previous starts this season, baffled the Orioles all evening.

Despite six walks and two wild pitches, Mazzaro turned in an effective start, holding the Orioles to a run on three hits over six-plus innings. It marked the 59th time in 76 games the Orioles have allowed an opposing starter to go at least six full innings.

"Luckily, we have a deep 'pen, guys that can step in -- like V -- and come in when called upon and put together a good job for us," said Crisp, who robbed Corey Patterson of an extra-base hit in the seventh. "[Mazzaro] did a fantastic job."

It didn't hurt that the Orioles were unable to capitalize on several key early chances, most notably Luke Scott's leadoff double in the second. With Scott on third courtesy of Mazzaro's wild pitch, Adam Jones hit a fly ball to right field that Rajai Davis caught and threw home to easily nab a standing Scott as he tried to cross home plate.

The O's stranded another pair of runners in the third, before finally scoring during a contentious fifth inning. No. 8 batter Julio Lugo was plunked on his upper left shoulder to open the frame, and angrily barked a few words at Mazzaro before flinging his elbow protector toward the dugout and trotting down the first-base line.

"I don't know if he did it on purpose or not," Lugo said. "I didn't like it, he hit me too close to the head. It about hit me in the face."

The 90-mph fastball -- which Mazzaro said later wasn't payback for Matusz hitting Barton in the top of the frame -- brought several Orioles, including Samuel to the top of the dugout steps. Home-plate umpire Fieldin Culbreth diffused the situation, warning Lugo and keeping both sides in their respective dugout.

"I was just trying to go in," Mazzaro said. "I had [Lugo] 1-1. I'm not trying to bag him, I'm trying to get him out."

Lugo got the last laugh, advancing to second on a wild pitch and scoring on Cesar Izturis' ensuing double in the left field corner. Izturis took the stance that he was hit in the foot with Mazarro's ball earlier in the count, with Samuel coming out to argue in support, but Culbreth ruled against it.

Izturis moved to third on Patterson's bunt to put the potential-tying run 90 feet away. But that's as close as the O's would get, as Tejada struck out for the inning's second out, and after Nick Markakis walked, Wigginton grounded out.

The Orioles scored again in the ninth off reliever Craig Breslow as Markakis belted a leadoff double and scored on Scott's one-out double. But, as was the case for most of Tuesday evening, the Orioles couldn't come up with the big hit. Baltimore went 2-for-11 with runners in scoring position, stranding nine men total and wasting a five-hit attack that included four doubles.

And, in the end it was Matusz, who paid the price, posting another fine performance wasted on another Orioles loss. In his last 12 starts, the O's have scored just 17 runs total with Matusz in the game, handing him a 0-9 record over that stretch. It's a tough burden for any pitcher, let alone a rookie. But Samuel said he's not concerned with Matusz letting the situation get to him.

"He's making pitches and that's all that matters for us," Samuel said. "He is progressing. He's making good pitches. We're not worried about him at all."

Koji Uehara, making his first appearance since being activated from the disabled list on Sunday, came on for the ninth. Uehara allowed a run on two hits -- including Crisp's leadoff double -- to give the A's an insurance run.