Granted I've worked only for smallish companies (<2000 people) but most places don't have specific policies _not_ to do it, especially if you do your due diligence security wise, and don't show the company in bad light.

If you make it a very nice looking OS project, most of the time they are either on board or don't care too much.

But I really check for this at the interview stage, and they must have a really good reason not to, compensated accordingly.

Didn't Apple start doing OS as well though? I think there are a bunch of projects from them already.