What fish start with the letter o?

To begin with the most obvious, ocean starts with O.

The octopus lives in oceans.

Some species of otters also live within oceans.

Orcas, which are commonly incorrectly referred to as "killer whales", are large dolphins which live in oceans.

Oysters are found in oceans as well.

There is also a predatory snail in the oceans called the oyster borer, which preys on, you guessed it, oysters.

Ostracod are small marine crustaceans, which are sometimes also called seed shrimp.