Both of those themes from Omori are beautiful. Haven't played the game though
I love this question; there are so many iconic video game OSTs out there! Man, just too many awesome soundtracks to begin listing. A lot of video game themes slap to this day, even if the game itself is mid.
A good example; the theme that would play during The Mercenaries mode in Resident Evil 6! The game was arguably mediocre (for a number of reasons, gameplay and story were both decent in their own right but didn't really mesh well? Pacing in the writing was off, things just didn't feel right, as far as a Resident Evil title goes)
BUT this theme still slaps. Perfect for HIIT (high intensity interval) training! The sprinting animations and the breaking of the 'bonus time' crystals always went great with this particular OST lol