With respect, there were not two endings of this movie filmed, and certainly there were never two different endings shown in different theaters. (And filming two different endings to a movie is really not at all common.)
The only ending is the one seen on the DVD, where O'Brien (the correct spelling of his name, by the way, everyone), having killed the leading baboon, is then beset by the others, in long shot, converging on him. It's not the cleanest ending, but clearly the others are about to kill him.
Besides, O'Brien was badly bitten by the alpha male before he killed him, and baboon bites are extremely toxic -- not poisonous as such, but so infectious that O'Brien would almost certainly have died soon after anyway.
I suspect, Fingaroo, that, as happens to most of us, your memory of a film you saw 40 years ago has simply become hazy and distorted, so that you think you remember something you never actually saw. I've seen this movie off and on for over 40 years and there is only the one ending...which is one of the more bizarre conclusions to a film I've ever seen.