I do a quick pencil drawing on heavyweight illustration board and fix it with workable fixative. Then I seal the surface with acrylic matte medium, applied quickly overall with a housepainting brush. While it's wet, I squeegee the matte medium to a whisper-thin layer using a scrap of cardboard dragged along the surface. Here's a video showing how that step looks.
When that layer is dry, I scrub in the oil colors with a bristle brush, thinning them with a combination of odorless mineral spirits and alkyd painting medium. The resulting layers take a couple of hours to dry to the touch, and they're dry enough overnight to permit additional layers.
What I like about the method is that you can use it entirely transparently, as I did in the painting above, or you can paint semi-opaque or fully-opaque passages, and bridge into a more conventional oil painting technique.
Book: Dinotopia, A Land Apart from Time (on Amazon).
Book: Dinotopia: A Land Apart from Time (signed from my web store U.S. only)
More about Dinotopia techniques and methods in my book: Imaginative Realism: How to Paint What Doesn't Exist
Listen to the latest episode of the Dinotopia audio podcast.
There will be an exhibit of Dinotopia originals at the Stamford Museum and Nature Center in Connecticut from February 14 - May 25, 2015. I'll be giving an illustrated lecture there on Sunday, February 22.