There's a fuckton of really fantastic mods for Oblivion, many of which dramatically improve the game; half the fun for me of an Elder Scrolls game is just in all the user-generated content.
This looks like a pretty robust guide, but I also want to specifically recommend
Lost Spires, which was a particularly good content mod that added more of the stuff I liked best - exploring. (And apparently they've added voice since I last played, which was my biggest problem with it.) There's also some impressive graphical upgrade mods, although those are probably no good for you unless you're running a rendering mainframe you stole from Pixar or something.
I'd also STRONGLY recommend some kind of scaling and/or leveling mod; the way it's set up in the basic game can get horribly, horribly wrong, and it's the biggest problem with the game. Fortunately, it's also a very heavily-modded element of the game. Unfortunately, some of those mods aren't good, and some are very complex. It's also a very personal thing, so you'll have to do the research on that one yourself.
For obvious reasons, you'll probably want some kind of mod manager.
Also there's
this thing, which sounds Very Interesting, but I haven't tried it.
