This game is indeed far better than fallout 3. IMO, fallout 3 is overrated. It's world is drab and glitches are through the roof. Furthermore, It lacks a sense of world balance that is present in pretty much every adjacent open world game. It also contains only 1 color pallet... and that pallet is shades of brown. If you want to expand on the universe, DLC is very disappointing.
Oblivion on the other hand is one of the best games I have ever played. It's world is perfect and the lore is both fantastic and diverse. I've spent literally HUNDREDS of hours playing and discover something new every time I boot up the game. And if you end up wanting more, Shivering isles kicks the shit out of just about any other DLC I've ever seen.
Seriously, considering you like fable, you're probably into fantasy stories/worlds. I don't see why you would pass this game up...
Also something to consider: Oblivion is the sequel to Morrowind... (Another one of my favorite games!) I suggest you play that as well. There's many references in Oblivion that can only be understood by playing Morrowind. It would enrich the experience IMO. It's not necessary though...
