Oakland led 6-1 after the fifth inning, rocking Tigers' ace Max Scherzer for eight hits and six runs in five innings.
Scherzer was bidding to become the first 20-game winner this year but instead needed the big comeback to avoid getting his second loss of the season.
Austin Jackson walked to open the inning and Oakland reliever Grant Balfour (0-3) got two outs before walking Fielder. Victor Martinez followed with a single that chased Jackson in and made the score 6-4 Oakland.
Hunter fouled off the first pitch he saw and the second was a ball. He connected with the third for a shot over the fence in left-center for his 16th home run of the year and the walk-off win.
Joaquin Benoit (4-0), who pitched a two-hit ninth, came away with the win.
Oakland had a trio of two-run rallies off Scherzer. Jed Lowrie hit a two-run home run in the first and Daric Barton and Eric Sogard added fourth-inning sacrifice flies. The Tigers got a sacrifice fly of their own from Bryan Pena in the fourth but Brandon Moss hit a two-run homer -- his 25th this season -- in the fifth of the 6-1 A's lead.