squid
鰞 centers on squid as a sea creature. The meaning is specific and concrete, with no major abstract extensions.
鰞 combines 魚 (fish) and 烏 (crow/black), likely suggesting a sea creature associated with darkness or ink, fitting the squid's nature. The exact historical development is uncertain.
A fish (魚) that is as black as a crow (烏) and squirts ink: that's a squid.
For オ, imagine an old squid with a long white beard, and you exclaim 'Oh!' (オ) when you see it.