No implementations provided for the following modules: Str

You almost had it ;-)

Str is not in the standard library so you need to tell oasis to take it. In your _oasis file just add one thing in your Executable part :

_oasis

Executable "golfcaml"
  Path: src
  MainIs: golf.ml
  CompiledObject: best
  BuildDepends:
    str

And it will work fine :

make
ocaml setup.ml -build 
Finished, 0 targets (0 cached) in 00:00:00.
Finished, 4 targets (0 cached) in 00:00:00.

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