Buck beat out the ground ball to drive in pinch-runner Rajai Davis from third and cap an inning in which the A's did not hit the ball out of the infield.
Lopez walked Jason Giambi to open the 12th. Giambi moved to second on a groundout by Mat Holliday and then strolled to third as Lopez walked the next two batters. Giambi was lifted for pinch-runner Davis, who bolted for home on Buck's chopper to the mound.
Oakland erupted for five runs in the first inning off Boston starter Daisuke Matsuzaka, who left after one inning, to take a 5-3 lead. Boston's bullpen kept the A's off the scoreboard until the 12th.
Mike Lowell notched one of his two RBIs in the fifth with a single that drove in J.D. Drew and tied the game 5-5. Drew had three hits and an RBI for Boston.
Senate Democrats to pull all-nighter on climate change
Interpol investigating stolen passports on missing flight