seagull
鴎 centers on a specific seabird: the seagull. The meaning is narrow and concrete, referring to the bird itself.
common gull (Larus canus)
northern fulmar (Fulmarus glacialis)
giant petrel (esp. the southern giant petrel, Macronectes giganteus)
Cape pigeon; Cape petrel; pintado petrel (Daption capense)
old sword fighting; sword drawing style or school from Shizuoka prefecture
long-tailed jaeger; long-tailed skua (Stercorarius longicaudus)
glaucous gull (Larus hyperboreus); burgomaster
black-headed gull (Chroicocephalus ridibundus); hooded gull
skua (any bird of family Stercorariidae, incl. the jaegers)
laughing gull (Leucophaeus atricilla)
herring gull (Larus argentatus)
chignon resembling a seagull's spread tail feathers (Edo-period women's hairstyle)
Caspian gull (Larus cachinnans)
parasitic jaeger; Arctic skua (Stercorarius parasiticus)
鴎 is the modern simplified form of 鷗. It combines 区 (possibly phonetic) with 鳥 (bird) to denote a seagull. The older form used 區 instead of 区.
The left side 区 looks like a coastal area, and the right side 鳥 is a bird. Picture a seagull flying over a coastal district.
For オウ, imagine a seagull letting out a loud 'Oh!' as it spots a fish: Oh -> オウ.