OAKLAND, CALIF. ? In the Oakland A’s clubhouse, Britney Spears tells the team, “Let’s go, boys.”

OK, so it’s not really Spears, just a larger-than-life cardboard cutout of the pop princess. And someone just added her plea in black magic marker.

Not a lot of teams would allow Spears to stay very long. But these are the A’s, and pretty much anything goes.

The Athletics have built a well-deserved reputation as baseball’s plucky upstarts: a mostly young, low-budget franchise that pushed the mighty New York Yankees to the limit the past two postseasons.

If sheer enthusiasm were enough, the A’s would win every game. Even after a recent loss to the Boston Red Sox, Gold Glove third baseman Eric Chavez admitted there was fun in the chase.

“It’s amazing how we fit so well,” left-hander Barry Zito said. “It’s like we’ve been around each other our whole lives.”

There were fears the A’s might have upset their chemistry because of the hard reality of free agency in the offseason.

Oakland lost first baseman Jason Giambi, who had set a tone with his easygoing style, quick wit and penchant for dotting his locker with autographed photos of his favorite pro wrestling stars.

A’s management wouldn’t budge last season on Giambi’s request for a no-trade clause, even though they had agreed on a $91 million, six-year deal. So 2000’s AL MVP snagged a seven-year, $120 million deal with the Yankees and bolted to New York.

And Johnny Damon, one of the most positive forces in the A’s clubhouse despite a mediocre performance on the field, took an offer from the Red Sox.

The A’s responded by banding even closer together.

“Any thought that this team was built around one player or two players doesn’t give the other guys a lot of credit,” A’s general manager Billy Beane said.

Giambi recently came back to Oakland for a three-game series as a Yankee. He was doing yet another pregame interview with a TV reporter when SPLAT! A’s second baseman Frank Menechino nailed him with a shaving cream pie in the face.

The A’s are known for those pies Greg Myers got one from closer Billy Koch when he pinch-hit a three-run homer to give the A’s a ninth-inning win over the Anaheim Angels.

But while the A’s have remained their happy-go-lucky selves this season, the results on the field have been mixed.

Oakland got off to a quick start, winning six of its first eight games to actually hold a brief lead over Seattle in the AL West.

Recently, however, they’ve run into trouble, going on a stretch where they lost seven of nine including this week’s three-game series sweep at home by the Red Sox. They were at .500 going into the weekend.

One reason? Ace Mark Mulder was on the disabled list with a strained left forearm and the rest of the pitching staff struggled.

Still, it’s better than last season’s start, when the A’s went 8-18. They went 94-42 the rest of the way to finish with 102 wins and claim the AL wild card.

Oakland was the first team in baseball to win 100 games after falling 10 games under .500 in the same season.

Probably for that very reason A’s manager Art Howe arguably the most mellow manager in the game has plenty of optimism.

“It’s still early,” he said. “We’re not even close to clicking on all cylinders.”

One phase of Oakland’s game that isn’t causing any problems is the long ball. They led the majors with 49 home runs going into the weekend. Thirteen A’s have homered and 75 of their 163 runs this season have come off home runs.

“I knew we had some power. But if you had told me we’d be leading the league in home runs? I guess I’m pleasantly surprised,” Beane said.

Working with the third-lowest payroll in baseball, Beane has somehow put together a crafty mix of talented youngsters and experienced veterans for the past three years.

And, even more amazing, they all get along.

Seeing the boys on his recent road trip in Oakland, Damon couldn’t help but be nostalgic, even though he’s very happy in Boston.

“There will never be a clubhouse like that,” he said. “I miss out on that fun every day.”