This work is part of the beta-testing phase of opam-bin.
This set of repositories is provided by Origin Labs for Debian 10.4 systems on amd64. They contain sets of 350+ binary packages compiled for 4 different versions of OCaml.
Here is the list of repositories, check these pages for list of packages and versions:
opam-bin, © Copyright 2020, OCamlPro SAS & Origin Labs SAS. <firstname.lastname@example.org>