For sure 1970. I recall watching right in front of me (from the west sideline) Jim Anderson pick off the pass on the last play. I recalled him throwing the ball into the east stands. Looking at the replay, it looks like he got the pick off a bounce off of one of the players (OU receiver was covered by two Huskers).
In hindsight, umm... he should not have run it out of the end zone and he was a little too close to the sidelines to make that underhanded throw. I think he was out of bounds then, but he looked like he was in bounds when he lowered the ball. Until he was down, the ball was live; however, the defense could not advance a fumble back then, but if a defender (returning an interception) fumbled, could the offense have advanced the ball?
I started going to some games in 1967, but I have no real recollections of the OU game. I think I sort of remember it, but it may have been OSU, and I am a little skeptical that I would have gone to a Thanksgiving game that year. Since 1968 & 1969 were both in Norman, and I recall 1970, I have to go with it.