Overall, a nice OST here! The compositions are fun and creative, you have an interesting style. The blending of piano with synths and chiptunes gives it the whole OST a unique vibe.
Stage Select has an interesting sort of chaos to it, but I feel like the piano rhythm drags on a bit - it would benefit from dropping out completely in part of the track or at the very least some velocity changes. The same can be said of the whole soundtrack - vary your velocity, everything sounds so stiff! Just because it's MIDI doesn't mean it has to be grid-locked and robotic. Throw some of that human element in there! If you can't play any instruments (I sure as hell can't) then learn how to humanize by studying MIDI that came from an actual person playing. It's a good way to see how the velocity (especially of a piano) will ebb and flow over the course of a performance. For some instruments some randomization can go a long way, just be careful not to throw off your song's overall timing by doing it TOO much.
I am loving "BOSS (the SqUaNCher)" - that little vocal bit was a really nice touch. Some songs feel a bit busy - "THE VOLCANO" is a guilty of this. You could maybe try dropping some of the drum notes to something simple for a couple bars around 0:36 and then bringing them back in later on to give this song a little room to breathe.
On track 8 - what the hell man, you didn't tell me you could play piano? I wrote all that stuff about velocity for nothing, and I'm not gonna go back and delete it either. You can't just throw this beautiful piano performance into the middle of a soundtrack like that without warning me. Sheesh. What are you even doing, this is like 5 minutes??? Chill out, bro. Show-off. (just kidding, nice work on this one)
Now you're throwing in some sort of funky guitar vibes in there with "THE CORE." I like it! I think this is one of the better executed themes on the soundtrack.
Anyway, pretty cool OST here. Nice work!