Thanks brab. I used #require which became available only after #use “topfind” .
So far code blocks below works.
#+BEGIN_SRC ocaml
#use "topfind" ;;
#require "owl";;
open Owl
#+END_SRC
and
#+BEGIN_SRC ocaml :results output
let m = Mat.uniform 2 2 in
Mat.(print m)
#+END_SRC