Ask HN: Are there any CTFs/wargames teaching operating systems concept?

Recently I've been "playing" the introductory CTF at overthewire and ctflearn, and I found that to be a good brushup of my Linux skills.

Soon at my technical uni I'll be having Operating Systems course that's widely considered to be one of the hardest subjects there.

With that in mind, I'd like to start learning about the subject in advance. And there comes the question from the title - do you know anything of that kind?