It's 4 hours since I've finished the game and I'm sitting here in the middle of the night, still completely entangled in the game's atmosphere, digesting everything by myself and no one to talk to about it.

If you haven't heard too much about this game it'll catch you off guard and that's just spoiling already too much.

I'm one of the lucky ones who haven't heard about OMORI at all, while some people surely were frustrated that it took almost too long until it finally released. Without researching the reasons for the long development time I think the game overall did profit out of it, since most of the reviewers here are more overwhelmed than being frustrated anymore. And it shows!

A little bit over the game overall:
Since I'm a sucker for those type of games (oldschool, SNES-like, unique graphic style, more than meets the eye) I was pretty fast interested in it after I saw the trailer here on Steam, hoping for a story that hopefully offers more than I expected at this point. I tried to avoid spoilers and gave it a shot.
To be honest, to keep it simple without going into detail, the first hours were pretty lighthearted and lowered my defense for what had to come (some people could maybe say that the pace at the start is a bit slow). This was kinda good since therefore the game could establish a growing atmosphere which (like I said) indicated was has to come later on without overdoing it directly in the beginning. In my perspective this gave the game an overall perfect pacing which you'll learn to approve of when you've finished the game by yourself. At the latest from a very specific point (maybe 2 - 3 hours ingame) you'll see where the journey may take you and you'll eventually keep playing.

What I enjoyed more or less:
Over the course of my playthrough I really enjoyed the different worlds and lighthearted humor besides the story obviously. The artstyle and the change of color pallets between different worlds mixed with an huge soundtrack have kept it enjoyable and fresh throughout the run. The boss fights were especially great. Some bosses were pretty awesome staged due to having stunning artworks and intense music what helped greatly in motivating me as the player to always push forward. The fights were fun, thanks to the "emotion mechanic" which are almost the same as elements in RPGs, but it worked far better with the concept of the game.
At the same time I have to critisize the fighting due to the lack of fast transitions of moves between characters which stretches every fight a little bit too much. You can change the text speed which speeds up the fight a bit, but that doesn't change the speed of the transition since this is not taken into account with this setting. You'll also have a problem to follow up what's going on when the text goes too fast so you have to settle with it.
I get the feeling that some parts could be cut shorter since they didn't provide much to the plot, but at the same time it helps creating atmosphere (if not overdone) what is absolutely important for a game like this.

My conclusion:
Overall it's fair to say that you need at least a bit of patience and need to play it with less pressure to fully enjoy the ride. If you like some oldschool looking games mixed with a fresh artstyle and a unique, emotional and brutally honest storytelling, this is for you. Keep it and hold onto it. I love it and surely won't forget that game so easily anymore.

PS (spoiler alert):
No wonder Toby Fox, creator of Undertale contributed an original track by himself even though it's an easter egg.