OAKLAND -- For infielder Jose Castillo this last month has been more unpredictable than the weather in San Francisco.

He began Spring Training as a member of the Florida Marlins fighting for a starting job. Then he was waived unexpectedly and left without a job. And now, just one week after being claimed off waivers by the Giants, he is the starting third basemen going into Monday's season opener in Los Angeles.

"It feels good because this team gave me an opportunity to play. I'm happy for the team and I'm for myself," Castillo said. "When I got here, I was told that I was going to get a chance to play and that's all I wanted. To be the starter, that's great."

Castillo knows all about the highs and lows of baseball. As a member of the Pirates in 2007, he was relegated to a utility role after starting at second and third base in his first three seasons with the team.

Upset about his role on the Pirates, the 27-year-old from Caracas, Venezuela, asked to be traded and was released after the season. Castillo hit .244 with 18 doubles and 24 RBIs in 87 games last season before joining the Marlins in December.

"I don't know what happened. Maybe [the Pirates] didn't feel like I could play and that was that," Castillo said. "It something that I've moved on from, but I wasn't happy with their decision."

Castillo enjoyed a solid campaign in 2006 when he hit .253 with 14 homers and 65 RBIs in 148 games with the Pirates. Castillo, who has primarily been a second basemen in his career in the Majors, said he feels comfortable playing third base.

"I played third base with the Marlins in Spring Training, so I feel good there," Castillo said. "In Venezuela, I played every infield position, so I don't care what position I play. I just want to play every day."

Castillo started at third base Saturday against the A's and went 2-for-3, slapping a sharp liner to left in the second inning and a single to right in the fifth. Defensively, he made one blunder when he ran into shortstop Brian Bocock on a pop fly in short left field. The ball dropped in, allowing a run to score, and he was charged with an error.

With Castillo starting at third base, manager Bruce Bochy announced that Rich Aurilia will start at first base in the season opener. Bochy said he will continue to shuffle the lineup, with Dan Ortmeier getting some playing time at first base and Aurilia at third.

"Although Ortmeier has been swinging it better, he's struggled a little this spring getting on track, so we're going to put Castillo at third and Rich at first," Bochy said. "We feel that for Opening Day that's the best team we can put out there."