The Sooners (31-20, 9-12) squandered leads of 4-1 and 5-4 on Sunday, allowing Oklahoma State to score four times in the top half of the eighth and once more in the ninth to salt away a come-from-behind win.
Oklahoma has now lost five of seven Big 12 series this season, with one series remaining, at Texas this weekend.
Oklahoma State came out firing right out of the gate in the series opener on Friday in Tulsa. Back-to-back-to-back solo home runs in the top of the first put Oklahoma back 3-0, and the Sooners were never able to overcome the deficit.