How can I write a description of an oil painting?What elements should I mention?

 It is important to know the date of the work, the era of the work will aid you in researching the piece. Is it oil on canvas or something else, panel, frescoe, etc. Is it representational (something that looks like the object represented) or abstract? There are different painting techniques depending on the era and style. Is the paint thick or smooth and clean like a glaze? Be aware of the play of light, is it dark and shadowy or bright and colorful. The size of the painting is also relevant, some works may cover a whole wall while others are more intimate. words to look up that will be helpful; Fauvism, narrative painting, Impressionism, compositional content, chiaroscuro, vanishing point, provenance.  Also a few artists from different times and styles: Vermeer, Goya,  Matisse, El Greco, De Koonig, Thomas Hart Benton, these  are a few painters of a wide range of  historical times and styles. The museum will have a brief history of the work posted with the picture that will aid you in your research. Youmight also point out the changes in emphasis in images and technique compared to earlier works.