Alabama was favored by around 6.5 points.
Oklahoma's defense forced 3 turnovers, and they scored 17 points off those turnovers.
Despite being outgained in total yardage (Oklahoma had just ~212 yards) , they made the critical plays when it mattered.
Their kicker, Tate Sandell, hit all three of his field goals.
On the final play, Alabama went for it on 4th down, but Oklahoma's defense held strong.
So yeah — not just a good upset, but a backbreaking one for Alabama, especially at home in Tuscaloosa.