Paints are usually fluids or semi-fluids with pigments in them. Oil paints are no different. They are a type of slow-drying paint consisting of pigment particles suspended in drying oil. Most commonly linseed oil. If you have seen an oil paint, you can see that they are a lot more viscous in nature when compared to water-based paints such as distempers, cement paints, emulsions, etc.
The viscosity of oil paints can be modified by the addition of a solvent such as turpentine or white spirit, and varnish can be added to increase the glossiness of the dried oil paint film. Oil paints have been used in Europe since the 12th century for simple decoration, but were not widely adopted as an artistic medium until the early 15th century.