The Rangers took a 4-2 lead in the fifth on Beltre's 23rd home run and Hamilton's RBI single that scored Kinsler.
Oswalt was pulled after giving up successive hits to Matt Carson and Plouffe in the sixth.
Rangers reliever Michael Kirkman, who had retired 10 consecutive batters over his past three appearances, couldn't keep Oswalt's runners from scoring.
Carson came home on Florimon's groundout and Span tied it with a single.
Hamilton's double in the first scored Kinsler and Andrus to give Texas a 2-1 lead. The 2010 AL MVP is hitting .345 with 17 RBIs in the past 16 games.
"Down the stretch, he is our horse," Washington said. "So is Beltre, so is Moreland, so is Murphy. We've got a pretty good team."
NOTES: Span was back in the lineup after missing the previous nine games with a sore right shoulder sustained while diving for a ball in center field on Aug. 12 against Tampa Bay. ... Minnesota DH Josh Willingham was scratched because of a stiff neck.