I'd suggest that, since OU is based on usage, then we don't say "the top x Pokemon are OU". I'd say "the Pokemon having at least x% of team usage are OU". It makes more sense that way. It would also mean that the OU list won't always contain the same number of Pokemon, but would increase or decrease depending on how many Pokemon are used by more than x% of teams.
I agree X-Act. But I don't like the implications...
I chose that any pokemon used in more than 5% of the battles would be called OU. The idea behind this is that if you make a team that manages to counter all of those pokemon, you automatically have a game plan for quite literally 95% of your battles. (granted, I could have chosen 10% as the threshold... but 95% seems like the default one).
http://www.smogon.com/forums/showpost.php?p=711853&postcount=7
With that in mind, this definition makes Froslass, Tentacruel, Crobat, and Ambipom OU. More or less, that means these pokemon cannot be used in UU battles if that is where the OU threshold goes.
I agree with Abamasnow. In BL battles, once hail gets going, we have no way of stopping it unless we decide to run a random Pokemon using a weather changing move which will just get cancelled out when Abamasnow switches back in anyway. In OU, at least it's balanced with "switch-in weather changers" like Tyranitar or Hippowdon.
On Smogon, there are no BL battles. There are only 4 kinds of battles:
1. Uber (Unbalanced)
2. OU
3. UU
4. NU (Unbalanced)
On Smogon, BL is the banned list for UU, much like Ubers is the banned list in OU. More or less, we don't care about how balanced BL is. We only care about the balance of OU and UU. Uber is only played because people sometimes like to play with banned pokemon, but it is generally accepted to be less balanced than OU and UU.
It is kinda strange, I know, especially because Shoddy has an option to challenge people for BL battles. But thats how it works around here.