To answer your question; most likely yes. Considering this is the official boards, it has the most users, both new and old.
I'd say you're probably right, it seems pretty normal to me that the official forums for the game will be most active. Of course, I don't know what other Oblivion related forums there are, and this is the only forum related to Oblivion and even other Bethesda games I'm a member of, so I don't know for certain, but it seems reasonable to assume that this is the case.
Since these are the official Bethesda forums, they most likely get more exposure than any other forums related to the same games, and players who want to discuss Oblivion or other Bethesda games probably come and register here even if they also visit other related websites. Not to mention that the Bethesda forums also attract players of Fallout 3 and New Vegas, who might hear people talk about the Elder Scrolls on the forums, and decide to try it, which can potentially bring new fans in, if they like what they see.