Gorgeous marks and colours! Also really fascinating to see how the varying opacity of the oil pastels adds so much interest here (lack of opacity is one of the things I find vexing with oil pastels!).
Triss, oil pastels are made by several different manufacturers, like any other medium, and so they will have varying degrees of opacity. It takes awhile to try a few different kinds, which also means financial outlay. A lot of artists will write them off because of the investment involved - and that doesn't even include finding a good paper you like!
Since they're all made with wax, oil and pigment, the cheaper, student grade ones tend to have less pigment and oil, but more wax, which makes them more opaque and drier - can be hard to work with. But they are great for a bottom layer. "Fat over lean" really applies to OPs! A layer of harder OP easily accepts higher quality ones, they will glide right over. Easy to build up layers this way, which can give you a really thick, nicely textured end result.
Senneliers - very high quality, lipstick texture, but several are so oily you can see right through them. Expensive.
Sakura Cray-Pas Expressionist - student/midrange quality, great first layer OP's with nice color ranges. Super cheap, too.
Mungyo Gallery Artist's Soft - my general, all purpose go-to OP's, they have really nice opacity for layering. They don't have great color longevity, so it's best to not make them a top layer.
I could go on, but you get the idea.
And I'm a sucker for OP's, I freely admit.