oyster
蠔 centers on the oyster, a shellfish creature. The meaning is straightforward and specific to this animal.
蠔 combines 虫 (insect/shellfish) with 豪, which likely contributes a phonetic element. The character is used specifically for oyster.
The left side 虫 indicates a shellfish or creature, and the right side 豪 suggests something grand or impressive. Picture a grand, impressive oyster with a pearl inside.
For コウ, imagine a coat (コート) made of oyster shells: coat -> コウ, and the oyster provides the material.