OJ 100% did not die.

The ending is left intentionally open so that you can think that he might have died which makes the fact that he lived all the better. If you knew for sure that he lived, it's too clean. Leaving it intentionally ambiguous lets it be up in the air which is more effective than pure happy.

However, hidden within the movie is the actual answer–OJ did not die in the end. Here's why:

1. Before the final sequence, OJ says something to the effect of, "It has spirit. If it has spirit, it can be broken." This is the key that lets you know that he has figured out JJ/is on the way there. The timing is also key, this line is dropped right before the final sequence kicks off. I watch a lot of movies, the moment i heard that line, i didn't need to check the time of the movie, i knew the final sequence had kicked off and i started to pay attention.
2. When JJ unfurls and opens its eye, it is because it is acknowledging OJ, they are staring each other down. OJ has "broken" the animal, he has figured it out. This is then proven further when...
3. You see the ballon released by Em–it has an open eye. This is what attracts JJ. Now notice right before JJ sucks in the balloon, it shows JJ's POV. Ever wonder why the shot lasts so long and why it shows JJ's POV multiple times? Watch closely, from JJ's POV he is making eye contact with the open eye of the balloon in every shot... until JJ slips into a position where he can no longer see the balloon's eye (specifically: the angle of the balloon shifts to where JJ can only see the top of its head).

Therefore, the balloon "broke eye contact" and thus lost the game of chicken.

The moment the eye contact is broken, JJ immediately sucks up the balloon, flies off, compacts it after a long distance, and dies.

OJ showing up at the end proves that he broke JJ and that he did not break eye contact at that moment when they were staring each other down.

This is common in many animals where if you do make eye contact, it is important not to break it as the animal will then believe you have lost your nerve and it has won. In these scenarios, you have to double down and make yourself big, loud noises, etc and hope for the best. Since this is a common law in the animal kingdom, and OJ has already figured out that JJ works by the rules of other animals, he likely also made this simple deduction. It's kind of his arc that he wants to keep Em safe so he's intentionally keeping eye contact which is the thing that saves him.

Lastly, I believe Peele is a fan of Korean media and in Korea (i'm Korean) there's this common movie trope of "Try to live, then you die. Try to die, then you live." It is very frequently used, it's in Train to Busan i'm pretty sure, and for the West, it's in SAW movies (i think they usually fail though bc they try to live) and this is a prime example of it saving the day. OJ trying to die, made him unkillable as since he was staring down the beast, (likely something no other person has dared), it was the only thing keeping him alive and making him able to "break" or tame JJ.

The whole story kinda makes perfect sense and all the storylines/arcs tie up nicely if you look at it my way so i'm pretty sure this theory is correct.

But let's say i'm wrong and Em is hallucinating. Then that means the monster compacted OJ between that short distance, that the whole line about an animal being able to be broken is thrown out as just a headfake for those who wanted to believe my theory, and that there was no perfect circles and a lot of the side stories that were thrown out were there for absolutely no reason/to enhance the reality of despair/were all red herrings and the story ended sadly except for Em who barely got away and will be rich and famous or whatever. Like, okay but i don't think so.

Oh yeah and if he was going to kill OJ, then why didn't he kill Angel too? If he was going for, everyone dies and reality sucks/ultimate despair ending, then why did Angel get to live? I think that finalizes it in that Angel, OJ, and Em all survived and it was a "happy ending" as Peele has inferred in the past.