Depends on the type of Orca.
Resident Orcas eat almost exclusively fish. Transient Orcas eat almost exclusively mammals.
Orcas are fascinating. Different pods have differently shaped dorsal fins. Some researchers believe the pods featuring the tallest and stiffest dorsal fins are populated by killer whales that from the youngest age spent their lives occupying the greatest water depth on average. When they travel or hunt, killer whale pods continuously cycle from the surface of the sea downward into the ocean depths and upward again to the surface to breathe, roughly tracing out a sine wave. Since killer whales spend the overwhelming majority of their time either hunting or travelling together with the other members of their respective pods, the whales associated with the deepest-diving pods tend to develop the tallest, straightest and stiffest dorsal fins overall. It makes perfect sense—as the deepest diving open-ocean predators age and grow, they subject their dorsal fins to less average net downward force over time. This is due to the fact that as depth increases, the compressive or squeezing forces present in ocean water, which act in all directions simultaneously and directly arithmetically increase in magnitude with water depth due to the steadily increasing collective weight of the water volume above, begins to predominate over the force the gravity, which remains constant regardless of depth, eventually dwarfing it. Over time, this fosters relatively unrestrained upward growth of the dorsal fin by placing considerably less average overall lateral stress on the structural tissues of the dorsal fins of habitually deep diving whales compared to whales populating other pods whose dives while hunting or travelling as a pack tend to be shallower and cyclically faster. Included among the latter are killer whale pods that tend to remain or linger closer to the coast, having adapted to local patterns of prey availability. Their dorsal fins tend to be shorter and somewhat stubbier. In extreme cases, such as with whales kept for long periods of time in captivity, dorsal fin collapse can occur, giving the fin a flopped-over appearance.