ONI Not Opening from Steam

I would suggest to remove the maybe corrupt savegame from saves folder, and try again. 

That sounds brilliant, how do I do that?

Users\username\Documents\Klei\OxygenNotIncluded\save_files

Locate the file named like your last uses base (or sort by date) and move it. If it runs afterwards, your save got corrupted. Please dont delete it, but post it here, because i think the devs would be very interested in such a thing.

You should also try to verify files integrity on steam to see if there's a corrupted file