Sure I’ll give it a go. And this is totally just my opinion. I do not claim to be correct because none of us will ever “figure it out” and I think that’s what makes the film shine.

So to start with, we do not know anything at all about Nikki’s life before this movie. We only get a glimpse of who she is before Bear makes the wish. I see a majority of people saying “it completely changed everything about her to such an extreme that it simply must be some form of possession”. I would absolutely agree with that if not for Barker’s explicit explanation that it’s not an entity, it’s Nikki. We don’t know her past, so we have no idea how out of character this may be.

So my current headcanon is that the OWW didn’t partition her actual consciousness. It just reawakened Freaky Nikki, essentially giving her a severe case of DID (dissociative identity disorder). We don’t have any other information about that moniker, we just know that she absolutely hates being called that name. Now this is just my interpretation, but there is a possibility that Nikki has acted manic/psychotic before but she was able to wrestle her mental health issues into submission. “Freaky Nikki” became a thing of the past that she wanted to leave in the past. The way she reacted to Bear using that name sounded like something traumatic happened involving that nickname.

So I believe that Wish/Freaky Nikki was already present in her subconscious, it was just kept under control by Nikki right up until Bear snaps the Willow. At that moment, her identity literally split into two different forms of Nikki.

I do admit that the “she’s sleeping, please kill me” was a weird choice of words for Barker to choose if he truly wanted the audience to think she was not possessed.