OS/2 is far more over than any of those, especially since they are all still being sold for new commercial installations.
In my opinion, this state of affairs has nothing to do with OS/2 itself, and everything to do with the applications. As far as the x86 platform goes, however, it's pretty much Linux, BSD, and Windows (well, and Apple in the coming months). I had the odd situation today where Linux was better supported than a proprietary OS (Solaris 10 x86) -- I've never had to deal with that before. PVCS and MQSeries are unavailable for Solaris X86, as is Oracle 9i.
While individuals may still happily use OS/2 (and there are a number of them here), the OS as a wide-scale commercial platform is pretty much done. I haven't even seen an OS/2 ATM in a while. :-(
Hell, I use still TOS 1.0 on a TT030 at home, but I'm not about to claim that Atari isn't "over". There are probably BeOS users out there as well. But neither of those could hardly be called going concerns.