The cost of externalities suffers from the evasion of "the commons" - nobody is directly responsible. 
However - ultimately it does not matter who pays directly. If the oil companies pay for environmental damage and clean-up that cost is passed on to us - their customers. If they do not pay directly then "the government" pays - with our taxes ! 
In either case WE pay - the oil companies do not print money nor suck it out of a safe at headquarters.
The arguments about who pays is a false debate - a diversion. WE pay in either case, either trough taxation or cost of product.