Oil pastels are sometimes under-rated as an art form.  Many of us have heard of or used Cray-pas brand oil pastels - they are fairly inexpensive and easy to use in laying down color.  Cray-pas were also the first oil pastels.  Cray-pas recently upgraded their line to separate student quality oil pastels from professional grade pastels.  There are many other brands of oil pastels, including Reeves Oil Pastels and Van Gogh Oil Pastels for student grade and Caran d''''Ache Neopastels, Holbein Oil Pastels, and Sennelier Oil Pastels for professional grade oil pastels.  I have tried many of these.  Oil pastels offer a great advantage over water-based media as they are very portable and in stick form.  I find them more convenient than colored pencil for laying in large areas of color, and much more convenient than chalk pastels because there is no dust.  Oil pastels can be combined with oil paints.   Mediums such as turpentine and Liquin that are generally used with oil paints can be used with oil pastels. 

Oil bars are different in the formulation and are not the same as oil pastels.  Oil pastels never harden, and Sennelier makes a special fixative to seal oil pastels.  This fixative is an vinyl resin base and is different from "workable" or permanent fixatives used for soft (chalk) pastels.  I have also used Lascaux Fine Art Fixative with very good results.  Oil pastels are a very permanent medium, as they are basically made from the same pigments as other paint forms, mineral oil, and wax, and the difference in grades of pastel are the quality of those ingredients.

Out of all the brands I have, I find my favorite to be Sennelier Oil Pastels. These were developed for and with artists Henri Goetz and Pablo Picasso by Sennelier and they are absolutely dreamy to work with.  Some of the oil pastels by other manufacturers are overly thick, so that when layering color, you are more likely to be shifting lower layers around instead of laying a new color on top.  Not so with Sennelier - layering is much easier to accomplish and there is very little residue in the form of clumps that sit on the surface, although thick layers can also be easily accomplished.

I use oil pastels for both finished paintings and when designing a painting that I plan to complete in egg tempera or oil paint.  Much of my illustration work is very detailed, especially my pen and ink work.  Oil pastels take up very little studio space and are a great way to "let loose" with color.  I love to use Sennelier''''s Oil Pastel Card - this is a stiff board-like paper with an excellent surface texture for oil pastels.  It is available in pads of many sizes with glassine paper in between the sheets, enabling the protection of the oil pastel painting.  Other surfaces for oil pastel are canvas, museum board, cold-press watercolor paper and even cardboard.  I tend to like using stretched linen instead of stretched canvas because of the somewhat knobby texture. One of my favorite surfaces for oil pastel is wood that is primed with traditional oil-based gesso.  These boards are usually used for egg tempera, but I find oil pastels really glow when on this surface.  Oil pastels are suitable for many different styles, from impressionist to more detailed work.