Generally the cat would have to be bathed multiple times with dish detergent to remove the oil.
If your cat has only ingested a small amount he may be fine. Just watch for any abnormal symptoms, and if these occur than he should see a vet immediately.
If your cat has ingested a lot of the oil than it would be best to take him to the vet for symptomatic therapy for a toxin ingestion, where they would give him activated charcoal to prevent your cat's body from absorbing the oil, IV or Sub cutaneous therapy, injectable GI protectants, and more.
We use diluted Dawn to wash oiled animals. Yes, just like the commercial, it's a real deal. You don't put the Dawn directly on the area, it has to be diluted to work well and then rinsed very well. With birds we know the oil is cleared off if the water starts to bead on the feathers again. Not sure about cats. We also use various treatments for toxins as well as watching for any burns to the skin.