He had two of Boston's 10 hits on a day when he and former teammate Manny Ramirez were named by The New York Times as two of 104 players who allegedly violated baseball's substance abuse policy in 2003.
The homer put Boston up 6-5 with two outs in bottom of the seventh, erasing a 5-3 deficit.
Jason Varitek also had three RBI, Mike Lowell and Jacoby Ellsbury one each, while both J.D. Drew and Kevin Youkilis added two hits apiece for the Red Sox, who ended a two-game skid.
The winner was Manny Delcarmen (3-2).
Closer Jonathan Papelbon recorded his 26th save.
Oakland's Rajai Davis and Tommy Everidge had two RBI each.
Bobby Crosby had one RBI while Nomar Garciaparra had three hits for the Athletics, who split the four-game series.
The losing pitcher was Craig Breslow (4-5).