Oil will not run out suddenly. It will be a slow, agonizing decline. As oil becomes scarcer its price will rise, and what used to be very cheap will become expensive. Society will be wholly transformed.
The reason for this is simple: a finite planet has finite resources. Once we have consumed all of our oil and other primary materials, there will be nothing left. Oil is not a renewable resource on any timescale comparable to its rate of consumption. The chemical reactions which led to its formation occurred over millions of years.
There are, undoubtedly, oil deposits that remain to be discovered. But the easiest have already been found and exploited. Each year, we consume more oil than we find. This is clearly going to cause problems.
It is not only a question of when oil will run out, but how society will change as it does. A world in which oil is much rarer—and therefore costlier—will be very different from our own. The modern petrochemical industry will have to change dramatically: everything from lipsticks to fertilizers and plastics of all types will either be made differently, or not at all. Transport will obviously become more expensive. This will spell the end of the West’s huge retail and supermarket networks, since these rely on road transportation and economies of scale. The price of imported products will rise, and international tourism will return to what it used to be in previous centuries: a luxury for the privileged few. Competition for access to the last remaining oil deposits will increase, and may lead to conflict.
These developments are inevitable, and will only be temporarily delayed by the current recession which is slowing down the global economy. Developing renewable forms of energy and reducing consumption are the two most basic measures we can take to prepare ourselves.