Although we talk about oil painting drying in the same way we talk about watercolor or acrylic paint drying, the process is different. With watercolor and acrylics, the paint dries through evaporation, that is the water in the paint is "lifted out" by spontaneously turning from a liquid into a gas, and the paint hardens. The hotter it is, the faster this happens.
With traditional oil paint, there isn't any water in the paint to evaporate. Nor does the paint dry by the oil in it evaporating away. Rather the oil oxidizes, that is it reacts with oxygen in the air which causes it to harden. (With water-soluble oils, the paint dries through a combination of the oxidization and evaporation.)
Oxidization may seem an unfamiliar concept, but it's what is happening when an apple you've cut in half turns brown (see Why Do Cut Apples Pears Bananas and Potatoes Turn Brown?). With oil paint it's not a process that turns your paint brown, but it does make the paint go hard. What we usually call "drying".
Anne Marie Helmenstine PhD, About.com's Guide to Chemistry, explains: "Oil paintings don't really dry in the sense that an acrylic painting or water painting would dry. Any organic solvent [such as white spirits or turps] in the paint evaporates, pretty much as you are applying the paint or within a couple of hours (depending on the thickness of the film). The rate of evaporation of volatile compounds will depend on atmospheric pressure, temperature, and humidity. Lower pressure, higher temperature, and lower humidity will all increase rate of evaporation of solvent.
"The linseed oil and pigments oxidize (react with oxygen) and harden, but the oil has a low enough vapor pressure that it doesn't appreciably evaporate. Cross-linking occurs between the relatively small oil molecules, essentially forming a plastic. This isn't really 'drying' since you don't have water evaporating off. Most of the hardening takes place in the first few hours/days/months after the paint has been deposited, but the process never really stops.
The process never really stopping is why you shouldn't varnish an oil painting as soon as it's touch dry, but should wait several months. The less time the oil paint has spent "drying", the more likely your varnish is too crack.
And next time you're impatient with the drying speed of an oil painting, how about distracting yourself by cutting up an apple and seeing if you can paint a quick still life before it oxidizes?