Mediterranean, Greek, Roman and Egyptian civilizations were who first used oil painting. Its made of vegetal essential oils. It was invented in the 15th century. Oil paint is a traditional paint choice. Becasue people have been doing it for so long, their are certain expectations, yet oil paitnign can be so versitile that you can explore new 'frontiers' of oil paitning. Also mixing is a large factor of oil paitning, and that on its own can be an art.
http://www.cyberlipid.org/perox/oxid0011.htm