One of the main appealing properties of Modern Oil Paintings are the glazes.  By adding a small amount of pigment to the relatively clear oil medium, you can very subtly tint an image.  This is called glazing. Most Renaissance Old Masters used a toned underpainting and then built up several of these thin glazes of colour on top to create astonishingly realistic figures and scenes. The translucence of the paint film allows for sophisticated ranges of flesh tones.

Okay, another analogy: imagine the top layer of oil is stretching as it dries out hardens, and it stretches to the max.  Its surface is expanding because it is absorbing oxygen. Now, they oxygen eventually begins to hit the layer below.  And it stretches and expands to the max. But they layer above is already dry, how can it expand any more with the one below pulling it!

Like a big cookie on a pan. Slide an uncooked cookie under a cooked cookie, a bigger one and stretch and heat up that dough: as the bottom cookie dries and expands its surface, it will crack the smaller cookie it is now stretching on its surface.To get around this, painters developed the Fat Over Lean rule. With each layer of glaze, add an increased amount of oil paint to the layer. This way, the rate of oxygen begin absorbed by an oily top layer will be slower than the hidden lower, less oily layers, and hopefully they will saturate with oxygen and harden at approximately the same time.

This leads to other tricks and techniques too.  If you use too little oil in an early glaze, it can obliterate the drawing or painting underneath that you want to show through all the thin transparent glazes. It also can make the paint too pasty and thick, which is unworkable for fine detail. So, in the early, lower glazes, sometimes solvents such as turpentines are added.

To answer the question above about the dandelion salad, oils themselves are not harmful. An open container of say, safflower oil on the table will do no more harm to breathe in than some extra-virgin olive oil with Balsamic vinegar and a few chili flakes on your table for bread.  It’s the solvents you have to be especially wary of. Even some of the odorless ones have harmful vapors, although it’s possible nowadays to buy non-toxic alternatives.

Related reading:Abstract Oil Paintings Oil Painting Reproduction