In the 90s, the relevant user-facing OSes for computers were MacOS and Windows.
There were at least real, meaningful attempts at competition in the consumer space with, most notably, OS/2 and BeOS. (OS/2 is seen as a commercial failure these days but quite a lot of people ran it.) MS-DOS from Microsoft was also competing with IBM PC DOS and Digital Research/Novell DR-DOS.
In the early 90s, Atari TOS, RISC OS and AmigaOS were not completely down for the count yet.
In the UNIX workstation market you still had SGI IRIX, Solaris, HP/UX, and more.
Now, looking even in that space -- well, maybe there's a Linux box or two, but more likely you're going to find even more macOS and Windows.
In the 2020s, the relevant user-facing OSes for computers are MacOS and Windows.
(However, to be fair, all the rage is now in phones, where Windows is surprisingly non-existent. Apple's OS is still the other one, though.)