OS is pretty hard, Prusa is moving away from OS and I wouldn’t consider them fully OS anymore. There are a lot of good projects around Voron community and I think RatRig is OS, but apart from that I don’t think there is a big OS project alive anymore. Saying that, there is a shitton of smaller projects, not full 3d printers, but motion solutions, extruders, electronic solutions, filament swappers and such, everything you need to build a 3d printer yourself.