I have always had trouble with making oil pastel show up as well as I wanted it to without taking forever on a project. And by the time I have finished pressing so hard, it's very difficult to add a tree or a bird or anything else next to or in the sky or any kind of landscape.
What you will need to make things easier is:
A sheet of paper
Step 1: Add Colour
To add colour without using up too much of a coloured pastel, go on with the colour not too heavily. Then, when all the colour is applied, take a white pastel and go over the colour very heavily.
Step 2: Make an Image
My sky is finished and I want to add a few birds. But there is no room, so I go in with the pencil after taking the eraser out and then etch out the image. Then, I go in with a colour to make my birds.