Oakland is 12-8, and just had a nice series against Cleveland where they had wins of 10-0 and 11-0 (in addition to a 6-1 loss in the middle). They have a huge test coming up this week with a 3 game series in Tampa starting Tuesday.
They've had some decent wins so far, including taking 2 of 3 against the Angels a few weeks ago. Their best series of the year, though, was a 3-game sweep at home against lowly Baltimore, so they still have a lot to prove.
They're 2.5 games ahead of the Angels, who most people expect to win the division. Unfortunately for the A's (and the rest of the division) the Angels usually play worse in April than they do the rest of the season.
Maybe this season is different. Go back 4 years: In 2006, the Angels led the division by 2 games on April 26th (following a trip to ALCS in 2005), and the A's ended up winning the division by 4 games.
In 2006, the A's had some power: Frank Thomas (39 HR, 114 RBI), Nick Swisher (35 HR, 95 RBI), even Eric Chavez (22 HR, 72 RBI). So far this year, Kurt Suzuki is their top power hitter (4 HR, 11 RBI), and there aren't a lot of contenders to take his place (except maybe Kevin Kouzmanoff).
The A's scored nearly as many runs in 2009 (759) than 2006 (771), but their lineup doesn't seem to have enough firepower to compete with the Angels. They have some excellent pitchers (Dallas Braden, Brett Anderson, Ben Sheets, Andrew Bailey), so it wouldn't be surprising to see the A's surpass their performance from the last few years.
Not only do the Angels have a more potent lineup, in addition to what should be quality pitching (the bullpen will probably be an issue), the AL West almost always looks different than people expected at the end of April. The A's brief reign atop the AL West is probably coming to an end pretty soon, although the 2010 A's pitching might be good enough to put them over .500 for the first time since 2006.
On April 26th:
2010 A's (12-8) ahead of Angels (10-10) by 2.5 G
2009 A's (7-10) 4 G behind Seattle (12-7), Angels (7-11) 4.5 behind Seattle
2008 A's (15-10) and Angels (15-10) tied atop the division
2007 A's (11-11) 1 G behind Angels (12-10)
End of Season
2009 A's (75-87) 22 behind Angels (97-65)
2008 A's (75-86) 24.5 behind Angels (100-62)
2007 A's (75-87) 18 behind Angels (94-68)