OS/2 is a lot easier in some ways to use than DOS for DOS games, as all the settings can be set through a dialog box rather than config files.
It can't achieve the impossible, though - more than 640K is granted in exactly the same way as through real DOS - using unused display memory, which in reality is useless.
It runs IBM DOS under a 16 bit virtual machine, with quite a lot of control. It's also possible if you want to run your own 16 bit real mode operating system (MSDOS, CP/M).
Ultima VII won't run, because it requires protected mode, and OS/2 is already controlling that. DOS/4GW apps are fine.
So, basically for some games it's easy to get them working, and others it's a tad more tricky. Obviously at the time it was amazing, as other programs would be running in the background at the same time as the game was running.
I used to be a huge OS/2 fan, but I'm not sure I'd install it just to run DOS games. I have it installed partly for that, partly for native OS/2 games (mostly Galactic Civilisations 2), and a number of native OS/2 apps.
With DOS 6, multiple configs, careful use of high memory, umbs, and mouse/cd drivers with low memory usage, it's not really difficult to meet memory requirements. When it was DOS 4, running on a 286 which couldn't use UMB memory, things were more fraught (I ran WC2 in that way, it was a tad slow)