The A will take more power (which you have) get louder, sound slightly better, and has 10mm more xmax. The O is newer, doesn't have the T-joint, and is guaranteed under warantee. Both are the same price within a few bucks...there's pluses to both sides of it.
One thing to keep in mind is that, on the O series or the rl-p, the warantee will be void if you're using the viper (unless you wire them the O to get 600ish watts). The rl-p is supposedly a great little sub that can take a ton of power, but you're voiding the warantee by hooking up more than 500 watts to it which you will do no matter what with that amp.
So now you're looking at 3 subs, one is impossible to have the warantee on with that power (rl-p), one may have a warantee if it's transferable (A), and one that you can have the warantee be honored if you wire your amp to give 600 watts (O).
Good thing is you've heard the O series, and you like the sound, that is something to keep in mind. Other than that, all the drivers I'm sure would suit you quite well, and the choice will come to the small details. The avalanche sounds like a very promising driver, but as others have stated, it needs a larger box so that is probably out.