mud, silt, clog up, obstruct
淤 centers on sediment that blocks flow: mud and silt that settle and clog, or anything that obstructs by accumulation.
淤 combines 氵 (water) with 於, which likely contributes the sound and the idea of obstruction or accumulation. The exact historical development of 於 is uncertain.
Water 氵 plus the shape 於 suggests something heavy settling in water. Picture thick mud and silt slowly clogging a riverbed.
For オ, imagine an old river clogged with mud: old -> オ, and the silt obstructs the flow.