If it would be true that oil is their only revenue stream, things would be a lot easier. However a german news article is estimating that it accounts only for like 28% of their revenue.
Das Finanzsystem des Terror-Kalifen: Der IS lässt sich nicht einfach aushungern - n-tv.de
They make huge money on plundered stuff and what they consider contraband in the areas they control. Like someone gets caught selling booze, they take all from him and make it money.
They even have a tax system in place, accounting for around 24% of their revenue.
Sure, these revenues will drop, once they sucked every penny out of the areas they control and there is nothing left to plunder, but it might take a while.