Houston lost 6-1 to the Seattle Mariners, ending its three-year reign as division champion and making the A's the first team in the majors to lock up a division crown in this pandemic-shortened season.
Oakland was off Monday before opening an interleague series at Dodger Stadium on Tuesday.
Since last season ended, the slugging A's have emphasized the need to win the division, given that they won 97 games each of the past two years and lost in the AL wild-card game.
Oakland is in the playoffs for a third straight year and the sixth time in nine seasons.
Houston won a major-league-best 107 games last season but lost to Washington in the World Series. The Astros brought in veteran manager Dusty Baker to lead the way in 2020.