Both oil and watercolor paint can be used to paint a variety of subjects, but some artists prefer one medium over the other for certain subjects.
Oil paints are often used for portraits and still lifes, as they allow for precise detail work and the building of texture. Watercolors are well-suited for landscapes and nature scenes, as they can capture the fluidity and organic quality of these subjects.
However, these are not strict rules, and many artists use both mediums for a variety of subjects.