Who are artists primarily known for using oil pastels?

Pablo Picasso was known for his use of oil pastels.

Many of my favourite Impressionist painters used oil pastels including Mary Cassatt and Edgar Degas.

Not to be rude to you, @muppetish, but oil pastels were apparently invented as a collaboration between Picasso and the Sennelier company which was already making dry pastels – this was in the forties, long after both Cassat and Degas had completed their careers.
The most interesting artist I’ve seen using oil pastels recently was
Richard Serra, who does a lot of drawing as well as monster steel sculpture. The pieces I saw were about 40×40 feet and completely covered with black marks so that there was no white space left at all. I know it sounds obsessive (it is) and almost as if there was nothing to see, but there was a subtle, insistent, aggressive texture to them and they were stunning emotionally. Who knew.