This seems like an interesting topic, so... Eh, why not? :3
Well, anyways, the story behind mine is pretty simple, really... It all started with my obsession with Kingdom Hearts. The concept of 'oblivion' is emptiness, an end, or a lack of memory. Now, yeah, that's kinda dark and grim, but I've always liked the word (I've got a habit of liking weird ones, like 'bizarre' and 'apocalyptic' and 'pneumatic') and I liked the characters in the games that are connected to it even more, so...
Bam. Oblivion was my choice of word. However, when I made the account, 'Oblivion' wasn't being accepted, so I plonked 25 at the end, that being one of the numbers I could guarantee to remember the best, as it's part of my birthdate.
Contrary to what some may think, I've never played the game titled Oblivion before, though... ^_^;
If I was to ever change it, I'd probably use another staple that's been in my 'repertoire' for even longer, which I use for my dA account and email. However, chances of most of the friends I've made on FH actually knowing to look for me on that are pretty slim... so I probably won't for a long time, if at all.
That said, even if it was kinda a random one, it's grown on me quite a bit, so I even use it for other things as well now!