/usr/lib/ocaml/ld.conf is in ocaml-base-nox 3.12.1-2ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2
/usr/local/lib/ocaml/3.12.1/stublibs /usr/lib/ocaml/stublibs