I've never played Oblivion, so no comparison with that, but I have played all the previous PC based Fallouts (1, 2 and Tactics), and FO3, though a fun game in it's own right, is a pretty poor RPG in comparison to 1 and 2, though the equal of Tactics (which also favored combat over role-playing).
Fallout 1 and 2 had better dialogue trees, more humor, NPCs actually had personality, low Intelligence meant your character could barely communicate (makes for a hilarious playthrough though most people just refuse to converse with such a "special" person). You could kill everyone in the world (bar your hometown in FO2), or complete the entire game without having to kill so much as a rat, bar the final boss. And the boss in FO1 could be talked into suicide.