I have "completed" Oblivion so far, never played Fallout 3 but I understand its the same thing. The cool thing about Oblivion is that you can do anything you want. You choose whether or not to follow any quest you are given, or even playing the main quest. After you load the game, you can ignore your "destiny" altogether and do whatever you want. If that means you want to collect brooms, or kill everyone, or go around making potions or eating all the plants you can find, it is up to you. In fact, with both of these games, after you complete the main quest, you can still play it and go about and do whatever you want. There are a bunch of side quests, and different factions you can join and such. And if that eventually gets boring, there are large modding communities for both games to add extra content into them. Be it new quests, factions, houses, boats, enemies, races, weapons, you name it.
The only thing that becomes a pain is if you start using mods is that your program folder can get very large!