Some of the pokemon in them is different. What makes a Pokemon in DW OU instead of just normal OU is that its only OU when it has its DW ability. Same thing how Blazekien's DW ability Speed Boost makes it Uber.
DW OU also allows for the use of Pokemon that have not yet had their DW abilities released or are not themselves released, such as Contrary Serperior and Genesect.
