postinst is in ocaml-base-nox 4.05.0-10ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | #!/bin/sh
set -e
if [ ! -e /usr/local/lib/ocaml ]; then
    if mkdir /usr/local/lib/ocaml 2>/dev/null; then
	chown root:staff /usr/local/lib/ocaml
	chmod 2775 /usr/local/lib/ocaml
    fi
fi
if [ ! -e /usr/local/lib/ocaml/4.05.0 ]; then
    if mkdir /usr/local/lib/ocaml/4.05.0 2>/dev/null; then
	chown root:staff /usr/local/lib/ocaml/4.05.0
	chmod 2775 /usr/local/lib/ocaml/4.05.0
    fi
fi
if [ ! -e /usr/local/lib/ocaml/4.05.0/stublibs ]; then
    if mkdir /usr/local/lib/ocaml/4.05.0/stublibs 2>/dev/null; then
	chown root:staff /usr/local/lib/ocaml/4.05.0/stublibs
	chmod 2775 /usr/local/lib/ocaml/4.05.0/stublibs
    fi
fi
for i in /usr/lib/ocaml/3.06 /etc/ocaml /var/lib/ocaml; 		\
do									\
	if [ -e $i/ld.conf ]; then					\
		echo "Removing leftover $i/ld.conf";			\
		rm -f $i/ld.conf;					\
		rmdir --ignore-fail-on-non-empty $i;			\
	fi;								\
done
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