A picture that is painted using oil paint is called an "oil painting". Oil paint takes a long time to dry. Artists find this useful because they can keep working on the painting for a long time. People say that Leonardo da Vinci worked on his painting of the Mona Lisa for four years, even though it is not a very large picture. Oils paints, and oil paintings are often just called "oils" for short. If someone talks about "painting in oils" they mean that the painting is done in oil paints.