I would like to bring attention to the one non-ou pokémon that I feel is the most deserving of ou status: abomasnow. seriously, people need to break out of the "hail stall is boring and not that great and all the teams are the same" state of mind. abomasnow can beat all bulky waters except for gyarados, can beat the broken latias if it lacks hp fire, can take out salamence/gliscor/dragonite/flygon with ice shard, and can leech seed+protect/substitute almost everything to create some real pain for your opponent. I know lady bug had a really good hail stall team that was quite successful, but there need to be more abomasnow users out there. its full potential in ou has not been explored at all. you don't need it to be a "true" hail team or a stall team, either!
I guess the main problems for Abomasnow are:
a) MASSIVE list of weaknesses (at least Celebi isn't SR weak and Fighting resist is really usefull). But if you need bulky waters counter - I don't think better one exists in OU (maybe Ludicolo can be a rival here).
b) "Hail only" mentality. People don't even consider it outside of Hail, which is pretty dissapointing, as I also see some potential in winter tree, especially on stall-like teams. Also STAB Ice Shard to fight dragons is really nice bonus to have for semi-stall or heavy stall teams. And it's hard to find usefull Ice Shard user, which is defensive or isn't UU (Mamoswine and Weavile for stall teams... yeah and Donphan is pretty outclassed Rapid Spinner). But I guess Abomasnow may rise, if Manaphy ends up in OU, as Abomasnow stops most sets cold (Signal Beam anyone ?)
c) Unfortunately SR weakness. SR is strong enough factor especially on something with mediocore statistics like Abomasnow to use it over Celebi or Shaymin expect for some small niche, which I mentioned.
d) It's the main problem - it counters ONLY bulky waters (however they're really popular, so it's worth a shot), and things he could check (dragons) is a problem, as he can't switch directly on any OU Dragon. Ok, he can also check bulky grass types like Celebi or Shaymin, but unfortunately they use these days HP Fire almost always, so winter tree doesn't stand a chance. I also must disagree with countering Latias - I'm not sure how Abomasnow can handle Draco Meteor from Latias anytime soon. Specs will slaughter him, Scarf also may deal quite heavy damage with Draco Meteor. Offensive Calm Mind also may give you few troubles. However, he can counter some versions, I admit it.
However it's interesting enough pokemon to think about using him. Maybe it's worth a shot to start thread in Stark about it ?