The circle drawings are made on the card board bases which some pizzas are sold with. Sometimes these are black. I have not yet found anything to do with these. The three shown were are in oil pastel.
The "oil" pictures are not oil pastels they are produced using "oilbar" which look like very large oil pastels but are something between oil pastels and oil paint.
Oil Pastel remains permanently rework-able, it never sets hard, for this reason it is best if it is 'box' framed, so that it can be protected from damage.
I recently experimented with oil pastel varnish but it left an obvious "slick" finish.
Also that very impermanence is the element of oil pastel which is most important to me.
At any time I can change the picture into something new, add something or completely re-work it.