We use it for everything, plastic, petrol, napalm, etc. Existing tech might well replace oil in all of it's many uses long before the stock runs out. What happens is, the oil doesn't just all disappear at midnight, see. It starts getting scarcer first. The price goes up.
People get more serious about finding ways to do without it.
The government finds a better way to kill people than napalm. People start making things to last again, to use less plastic. If gas gets expensive enough, those monorail systems will make sense. Some places are a heck of a lot closer than others though. Social democracy can work if the proper measures are put in place.