stagnant water, impure, filthy
洿 centers on water that is still and foul: stagnant, impure, or filthy. The idea is of water that has lost its freshness and become dirty.
洿 combines 氵 (water) with 夸, which likely contributes the sound. The exact historical development is uncertain, but the water radical fits the meaning of stagnant or impure water.
The water radical 氵 beside 夸 suggests water that has been left to sit and become foul. Picture a puddle with a boastful 夸 shape floating on top, too proud to flow, so it stagnates.
For オ, imagine an old, stagnant pond where the water has turned a murky ocher color: ocher -> オ, and the pond is filthy.