One of my favorite mixed media combinations is oil pastel and watercolor. I love the effect of mixing the two - or rather the way that the oil and water never mix - it creates such an interesting and unique look.
There is no wrong way to play with these two materials together, but I usually go in this order:
First create your drawing/ mark making with oil pastels. The wax in the oil pastels will act as a relief.
Now take your watercolor and paint over the oil pastel. The watercolor will not affect the oil pastel
You can also use your oil pastels on top of your watercolor! Either way, the magic is that the two mediums will not mix and blend!