Oscuro's Oblivion Overhaul is one of the best mods for Oblivion, in my opinion.
You can download it from Oscuro's site, Planet Elder Scrolls or TES Nexus. The initial download size is not that big, only 23 MB for the latest version (1.33), although the installer then downloads the actual mod, which takes up approx. 1 GB of space, from what I remember. Here is an installation and quick-start guide.
I would also recommend installing the Unofficial Oblivion Patch (compatible with OOO), as it fixes thousands of bugs left over from the official patches.
To quote Raven Dreamer:
Don't forget the unofficial oblivion
patch supplemental. The UOP introduces
a major bug regarding the Fort Sutch
Oblivion Gate, but the supplemental
fixes this, among other things