Your code is totally imperative. In some cases (probably in most) it's faster, but this not the best way to use OCaml :) Here is my solution in functional style:

let print lst =
  List.iter (fun l ->
      List.map string_of_int l
      |> String.concat " "
      |> print_endline
    ) lst

let rec permutations result other = function
  | [] -> [result]
  | hd :: tl ->
    let r = permutations (hd :: result) [] (other @ tl) in
    if tl <> [] then
      r @ permutations result (hd :: other) tl
    else
      r

let () =
  permutations [] [] [1; 2; 3]
  |> print

Your code is totally imperative. In some cases (probably in most) it's faster, but this not the best way to use OCaml :) Here is my solution in functional style:

Printing the list of lists can be done by iterating over list of lists:

let print lst =
  List.iter (fun l ->
      List.map string_of_int l
      |> String.concat " "
      |> print_endline
    ) lst

Next recursive function does:

• Selects head element of the list and makes it heading element of the resulting list
• Recursively calls itself on the list of all previous elements (minus resulting subset) + tail.

let rec permutations result other = function
  | [] -> [result]
  | hd :: tl ->
    let r = permutations (hd :: result) [] (other @ tl) in
    if tl <> [] then
      r @ permutations result (hd :: other) tl
    else
      r

All together. Initial result is empty and the stored list of previous elements is also empty:

let () =
  permutations [] [] [1; 2; 3]
  |> print