First, I laugh at you girly-men who can't take the two tests on the same day.
Now, from what I remember the OS exam seemed to have a lot of WinNT questions (either installing it or upgrading to it or upgrading FROM it).
I believe that it is very important to do a complete reinstall of Win98, WinNT and Win2K from an empty hard drive (know how to format and FDISK the drives too). You should also know the main registry keys and there general purpose and the main system files for the different OSs (Command.com, io.sys, win.ini).
The best way to learn this is to know one install inside and out (say Win98). Then, learn the differences when you install Win2K and WinNT.
I don't remember seeing any "What do you have to click and what tabs do you select to get to this screen?" questions but that doesn't mean there won't be any.
I've never used/seen WinNT but I was able to pass with a score of 900 (higher than my core!) just by applying what I know about Win2K (since Win2K and WinNT are similar). I also knew the major differences between WinNT and Win2K.
Good luck.