Welcome to Fluther.
Before anyone can properly answer question, you really need to restrict its domain.
Are you talking “personal computers”, “computers” in general (meaning mainframes, minicomputers and supercomputers as well), “computing devices” (which could include telephones, iPads, iPods and so forth) ... and even some machines, I suppose. I know that steam boilers have “operating systems”, though you may not ever consider them. By the same token I’m sure that ships, submarines, locomotives, aircraft and other complex devices have “operating systems”.
There are more operating systems than you have even dreamed.
But I expect that you mean “personal computers”, and the first two responses are probably good enough.