Since oil pastels require layering of colors on the top of each other, how am I supposed to see the underlying sketch?
If you're able, a Tracer projector could suit your needs. You place it over a small sketch and it magnifies it. No digital images required. Any other kind of normal digital projector will work too - although the resolution might not be as good. Overhead Projectors (these) are great too, either sketch onto a transparency, or you can buy special transparencies to print on. You can often pick these up very cheap, or even for free - many schools have piles of them in my experience.

Any of these methods will allow you to project your sketch over the top of your work, meaning your sketch will not be obscured.