Not sure which type of oil you are referring to?
But if it is fine art, and not painting a house.....I would say, "fat over lean". Reason being is that you don't want the bottem layers of paint to dry slower, then the top layers. It will result in cracks appearing over time.
There are many things to consider, when painting with oils. Some like to paint with just straight colour out of the tube. Others like to use a medium, such as turpentine or linseed oil. It makes it easier to handle the paint, for blending etc.
The choice of support to work on. Stretched canvas, wood panel....how it is gesso'd. Also I think learning the property of the paint themselves help, whether they are transparent or opaque in nature. Transparent colours are good to use for undercoatings. Where opaque colours are good on top layers, depending on painting style.
Knowing the colour wheel helps. What are the primary colours...red, yellow and blue. You can create other colours from them. Complimentory colours are good to know. Often instead of using black to darken, just use the complimentory colour to darken with...eg yellow's complimentory is violet.
Last thing I will comment on is, don't use a pencil (graphite) to do the preliminary sketch. Charcoal is better, as graphite can leach through the paint, over time. Almost a strike through effect.
Sorry for the long, book like answer here.
It's hard to stop....lol