what is the working knowledge of OCaml you expect for a 3-starred project (i.e. medium difficulty)?
Don't take the number of stars as a very precise indicator. If you already know a bit of OCaml, my advice is to pick a project in a field you are already a bit familiar. So you will be able to improve both your OCaml skills and to learn new things in a field you already know a little bit. If you don't know OCaml at all, pick a field you are already very familiar with. If you are already a good OCaml developper, simply pick a project in a field you want to know more about. Usually understanding complex protocols and systems requires much more effort than improving some OCaml skills.
If you are already familiar with OCaml, and If you want to know more about Git internals and you already know a little how GC works (or if you already know about Git but you want to know more about GCs), the GC for Irmin would be a nice project. Feel free to ask questions on that list of on the Irmin bug tracker[1]. One of the first step would be to read how the Git GC works, and see if you/we can try to do something similar in OCaml - and then later try to integrate this to ocaml-git[2] - and finally integrate this properly into Irmin.
In summary: try to connect with things you already know and like, otherwise trying to learn both a new language and new protocol/systems might be a bit a challenge.
Thomas
