Who was DeNiro yelling at in The Deer Hunter?

I’m gonna spoiler this just to be safe.

Near the end of the movie, DeNiro and the guys are out on another deer hunt. After tracking down a deer and moving into position to shoot it, DeNiro instead lets it go, and then yells, “OK?!?” to the wilderness around him.

To what or whom was this line directed? I suppose it’s open to interpretation, so interpret away, because it went right over my head.

The deer.

I took it that he had taken a new outlook on life and death. And that moment was when he realized that he wasn’t the same ole Mike that shot the deer at the beginning of the movie. I never thought too much on it though.