filthy, dirty, impure, polluted
汙 centers on contamination and impurity: something that is filthy, dirty, or polluted, whether physically or morally.
汙 combines 氵 (water) with 于, which likely contributes the sound. The water radical suggests the idea of liquid impurity or dirtiness.
Water 氵 beside 于: imagine a clear stream becoming filthy and polluted as it flows past a dirty area.
For オ, picture an oily film spreading over the water: oil -> オ, and the water becomes dirty and impure.