Stumbled across this today - TriOS is a proposed operating system that is completely based on objects at all levels. Files, users, etc., are all objects that are persisted to disk on shutdown.
It's an extremely ambitious goal, but the ideas presented in their Technical Design Paper are pretty interesting and appear to be the product of quite a bit of thought. The Object Storing System is cool.
I've never designed or built an operating system, so I have no understanding of whether or not this is feasible. Given that major companies take years to build operating systems, I'm not betting on installing this anytime soon, but I like the concept.