The oil painting is a painting painted with oil paints / oil colors, in other words a medium is the oil color / Paint. Oil colors / paints contain pigment which is bounded with oil. It can be sunflower oil, linseed oil, poppyseed oil, walnut oil and many other natural or even synthetic oils. Oil paintings are painted usually on canvas, wooden panel, less often on paper or cardboard.
According to Giorgio Vasari an inventor of modern oil-paints was Flemish painter Jan van Eyck (1390-1441), see examples of his works: . Many disagree with this thesis, however Jan van Eyck deserves to be credited for a contribution to the oil painting technique and its development.
The surface which is to be painted with oil paints need to be especially prepared, for instance with an oil color primer. Nowadays the most popular oil primer is Gesso by Talens, however many artists buy primers produced by other brands such as Winsor&Newton, Maimeri, Schmincke and many others or even make them themselves. For instance I made the primer myself many times and found it as a very interesting experience.
Oil painting is considered as the most expensive painting technique due its complicated painting process, a time that paints need to fully dry and material costs.