The uniform of the Party is the blue overalls that are worn by men and women alike. The overalls are worn by both inner Party members, like O'Brien, and outer Party members, such as Julia and Winston.
The overalls have a number of purposes. Because both the very powerful and privileged inner Party members and the oppressed outer Party members wear the same uniform, this creates the illusion that everyone in the Party is the same. In fact, nothing could be further from the case: O'Brien lives at a level of luxury almost incomprehensible to people like Julia and Winston.
Second, the overalls are ugly, helping to dehumanize Party members. They also desexualize Party members, helping to reinforce the idea that sex is not for pleasure but for procreation.
Further, the uniforms very clearly distinguish the Party members from the proles. This can make it dangerous for a Party member to be in a part of the city Party members don't usually go: they can easily be spotted by Patrols or secret police who might question such unorthodox behavior.
In the end, the shapeless, ugly uniforms are part of the inner Party's plan to keep the vast bulk of the population in poverty. Instead of spending money on making life better for the average person, the Party engages in endless wars that consume resources without improving the standard of living. The Party fears that if people are comfortably well off, they will demand more rights.