Setting aside the general questions of visual composition – subject, form, balance, etc. – oil painting discussions should include texture, color mixing, brushstroke, and application processes, among other things. There is a concept called alla prima that concentrates on how the oil painter applies color to the raw canvas to produce shading, depth, and surface variety by using the properties of oil paint in its basic form, and by painting directly right into the raw paint, in one session, “wet into wet.” Because of oil’s unique properties, the usually unnoticed subtleties of shade in real life can be captured by oil’s ability to shade via thinness and thickness of paint, so that shadows, never black, can be shown as varying shades of the base color. Al Gury’s seminal book, Alla Prima, is an invaluable source, not only for the painter, but also for the observer and critic of oil painting. The palette knife, the brush shape and composition, and the quality of the oil paint medium are all factors in looking carefully at oil paintings. Your description, then, should include all these elements.