at, in, on, as for
於 centers on marking a location, time, or topic: it expresses being at, in, on, or as for something. Its core idea is pointing to a specific reference point.
於 originally depicted a bird, but its modern use as a grammatical particle is a phonetic loan. The exact historical development of the right-side component 仒 is uncertain.
The left side 方 suggests direction or place, and the right side 仒 looks like a person pointing. Together, 於 points to a specific place or topic: at, in, on, or as for.
For オ, imagine pointing at an object and saying 'Oh!' when you find it: Oh -> オ, and 於 marks the spot.