OAKLAND, Calif. — The home clubhouse at the Oakland Coliseum was not a place for claustrophobics Monday night after the A’s game against the Texas Rangers. Oakland clinched a wild-card berth with a 4-3 victory and the players packed closely together, shoulder to shoulder, hip to hip, spraying Champagne for far longer than their supply should realistically have held out. The circle they formed slowly collapsed on itself in a heaving mass of celebration, quite a contrast from the grandstand outside.

The fans who showed up for Monday’s game — all 21,162 of them — were suitably raucous, understanding what was at stake and roaring hearty approval throughout the contest. But the crowd was also conspicuous for the seats it did not fill. Oakland manager Bob Melvin is fond of saying that 15,000 fans in Oakland are as loud as 30,000 fans anywhere else, and he may well be right. To judge by the energy in the crowd, it was clear that this team was on the cusp of the playoffs; to judge by the numbers present, one could be forgiven for mistaking the proceedings for a midseason game against Kansas City.

Oakland’s victory was the latest in an expectation-defying run. And if the A’s win today against the Rangers, they will be the improbable champions of the American League West. The team’s attendance Monday, however, was par for the course. Although the A’s are in their first meaningful stretch run since 2006, they have failed to draw as many as 22,000 for any game in the current, and quite dramatic, homestand.

Oakland’s stadium is defined by the tarpaulins that always cover the third deck, and by the architectural monstrosity known as Mount Davis, a monolithic eyesore in center field named after the late Oakland Raiders owner Al Davis and built to add luxury boxes and additional seating for the eight regular-season NFL games that are played here each year.

A good spin doctor might point out that the A’s have improved their attendance every season since 2009, but that would serve only to mask the reality that the team finished last or next to last in that category every year during that span, topping out just north of 20,000 per game this year, with a low crowd of 10,054 in its fifth game of the season.

In effect, the A’s have held a pennant race and not too many people came.

“I’m not into telling people how to spend their money,” Melvin said before Monday’s game. “It really doesn’t matter how many fans we get here — we always feel them.”

Third baseman Brandon Inge, out for the season after dislocating his shoulder Sept. 1, put it a bit differently: “Smaller numbers, bigger hearts.”

Drummers in the left-field bleachers increase the overall volume at A’s games, and the unabated concrete does not hurt when it comes to shooting echoes around the place. The crowd’s roar Monday, and it was frequent, could easily have been mistaken as having emanated from twice as many throats. Still, compared with pennant races in places like Texas (the Rangers averaged better than 45,000 on their just-completed seven-game homestand), New York (the Yankees drew nearly 43,000 per game against the A’s in mid-September) and across the bay in San Francisco (the Giants have sold out every game since Oct. 1, 2010), Oakland’s intimate gatherings offer the general impression that local fans simply do not care as much.

Losing has something to do with this, as the A’s had not finished above .500 since 2006. There is the slate gray drabness of the cookie-cutter stadium, not to mention the negative association with the Raiders, whose fans have long held a reputation for abusive behavior. There is the team owner, Lew Wolff, who has alienated fans for years with a perpetual clamoring to move someplace else. (He already has a site in San Jose in preparation for the moment that Bud Selig, baseball’s commissioner, rescinds the Giants’ territorial rights to the region.)

“I’d like to thank the fans for being here … when they come,” Wolff told reporters in the victorious clubhouse, as Champagne sprayed around him.

In some ways, the modest attendance and the closed-off sections of the ballpark, which is officially known as the O.co Coliseum, only add to the underdog image of the team itself. After General Manager Billy Beane traded his three best pitchers — Gio Gonzalez, Trevor Cahill and Andrew Bailey — in the off-season for unproven rookies and minor leaguers, it was widely assumed that he was giving up on 2012 while collecting chips that would mature in time for the team’s hopeful relocation to somewhere else.

His moves, however, gave the A’s an immediate influx of new talent: Josh Reddick (a 30-homer hitter and anchor of the batting order); starting pitchers Jarrod Parker (13-8, 3.47 earned run average) and Tommy Milone (13-10, 3.74 ERA); reliever Ryan Cook (14 saves and a spot on the All-Star team), and starting catcher Derek Norris. All are 25 or younger. None have postseason experience. And to a man they are loving what is going on with their team right now.

“Our fans have all these signs, and you see them dancing every time something comes on the sound system,” Reddick said. “They get into it. They’re doing the Bernie” — a Southern dance craze that has been adopted by the team.

“And when Balfour comes in, they’re doing the headbanging, heavy-metal stuff,” Reddick added in reference to the team’s closer, Grant Balfour. “They’re into it for everybody.”

Forty-seven of the team’s 93 wins have come from rookie pitchers. The A’s have struck out a league-record 1,372 times but lead the majors with 110 homers since the All-Star Game.

It all makes for a potent combination — the unlikely team whose winning ways have been appreciated by a select, but clearly enthusiastic, group of fans, although that number will surely grow when the postseason arrives. For now, it is odd and sort of endearing.

“I’ve been coming here since my mother was pregnant with me,” said Mat Moorhead, 29, of nearby Antioch, Calif., who is one of the drummers in the left-field bleachers. “My grandfather came here for games, my parents came here for games, and now I come here for games. I’d like to think that more people will show up … but honestly, I’d rather have 15,000 people in the stands who want to be here than 30,000 who don’t care as much as we do.”