OU means over used. There are some tiers. Ubers (banned pokémon unless you play ubers) OU (over used, this is standard battle) UU (under used) NU (never used) NFE (not fully evolved).
When you play in one tier you can use all pokémon in that tier and the tiers below. For example, if you play OU you can use OU, UU, NU and NFE (but NFE would be too risky since they are less powerful, you know...)
You can check the tiers here.
http://www.smogon.com/dp/pokemon/