Oil painting is done on surfaces with pigments ground into a medium of oil - especially in early modern Europe, linseed oil[?].
It was probably developed for decorative or functional purposes in the high Middle Ages. Surfaces like shields - both those used in tournaments and those hung as decorations - were more durable when painted in oil-based media than when painted in the traditional tempera paints. Many Renaissance sources credit northern European painters of the 15th century with the 'invention' of painting with oil media on wood panel - Jan van Eyck often mentioned as the "inventor".
Recent advances in chemistry have produced modern oil paints that can be used with, and cleaned up in, water. These are still 'real' oil-paints in every sense of the meaning. Small alterations in the mollecular structure of the oil creates this water mixable property.
