Dung beetles make some oil of ya give them poop.
Tame beetle+poop=oil

Whales make a lot of oil just make sure it has plenty of raw FISH meat

Mining drill and a good wyvern is my favorite way the shiny black rocks along north coast and on icebergs on the island will yeald decent amount of oil and quickly much better then swimming to bottom of ocean

basilosaurus once tamed will produce oil

Baliosaurus farming is the best

Killing tek creatures gives you oil.

There are oil rocks at the bottom of the ocean in Rag. Just make sure to watch out to sharks. Also, if you can find an oil rig in a drop, much easier harvesting.

There are many ways to obtain oil. My personal favorite is, to run around and tame a few Tek creatures. Kill their babies or the wild ones and harvest them with a chainsaw, if you don’t have one use a hatchet. If you need oil over a long timespan just tame a few Tek creatures an let them breed. Kill their babies and youll get a lot of oil and some great side products like meta scrap, wich is a great equivalent to metal. If you don’t have the time or nerves to do that you can just go to the ice biome and destroy the big black rocks (our intended) and youll get a lot of oil in a short time (the only problem is the transportation of yourself and the oil from the ice biome to your base)