Cahill gave up a lead-off single to Marlon Byrd in the second inning and the Rangers managed only two base runners the rest of the way.
Texas has lost four straight games and has been shut out in three of them.
The Rangers were six games behind the American League West-leading Los Angeles Angels when play began Wednesday and they were trailing Boston by 5 1/2 games in the wild-card race.
Cahill (9-12) walked two and struck out seven in a seven-inning effort. Michael Wuertz and Andrew Bailey each worked a perfect inning of relief.
Scott Hairston doubled home a run in the first and Oakland added three more in the sixth, one of them on a homer by Landon Powell.
Dustin Nippert (5-3) suffered the loss after allowing four runs on five hits in 5 2/3.
Copyright 2009 by United Press International.
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