In the top of the eighth — with senior Jake Fisher pitching in relief of Copping — Hofstra's Austin Nyman connected for a two-out, two-run double to bring the Pride within one run.
Nyman stole third base, and designated hitter Bryan Verbitsky's infield single drove him home to tie the game.
OU sophomore Robert Tasin earned the win after pitching a perfect ninth inning to set up Oberste's heroics.
Hofstra (0-4) pushed Oklahoma in the series' final three games. The Sooners won Friday's opener handily, but sneaked by Hofstra 3-2 and 11-10 in a doubleheader Saturday, needing a huge rally and extra innings in the second game.
“We won every game in every way possible,” said senior catcher Max White, who was 3 for 4 with two RBIs Sunday. “We beat them pretty good, then we had a close game, had to close it off. Then we came back, then we just won it like that.
“That's all the ways in baseball you can win.”