They say "you don't always remember what people did, but you remember how they made you feel." Oni is one of those games.
I remember playing in my "home office" in our first apartment late at night, when I needed a break from project work. Bits and pieces stayed with me, like the warehouse level where I know I kept dying and took a while to get through. But like so many of us have experienced, people move. We pack what we want and discard what we don't, and sometimes things fall through the cracks. My copy of Oni fell through a crack some 20 years ago and when I look at the physical copies which did make it - Medal of Honor, Activision Anthology, Quake III Arena, UT - I really wish Oni made the trips too.
I don't remember too many specifics, but playing it made me feel great.