Sizemore then belted a pitch into the right-center field gap, chasing home Pennington with the winning run and securing the Athletics a split in their doubleheader with the Angels.
Los Angeles won the first game 4-2.
Brad Ziegler (3-1) got the victory after hurling a scoreless top of the 10th. A's starter Rich Harden held the Angels to three runs on four hits over seven innings.
Mark Trumbo cracked a two-run homer and Vernon Wells added a solo shot for the Angels.
Exploding whale video goes viral on Internet
Texas principal bans speaking Spanish, stirs controversy