if you're saying windows and OS/2 are similar then there is only one operating system in the whole world i am afraid.
many of the systems i am sure people think of as 'unix' are not in any way the same as each other. BSD is not Linux. Linux and BSD are not minix. Minix and linux are not solaris. Solaris, minix, BSD and Linux are not UNIX. None of these is AIX, none of them is any of the other myriad unices out there.
'isn't x similar to y though?' i hear you ask... well you know what? to the user all those unices are almost identical, but internally they are totally different. their similarity to each other is one of their many strengths, but many of them contain NO CODE IN COMMON. they are TOTALLY UNIQUE from the point of having parts of the system the same.
So what makes the system? the name? the GUI? the fact that the CLI is similar? there are hundreds of completely different systems out there, depending on how you look at it, or else, as was said before, there's only one.