Trouble with multiline if statments in OCaml

I wasn't able to find a clear answer on Google, but it seems multiline if-statements are discouraged in OCaml (?) The ones I see with more line seem to have begin end keywords in them.

I'm currently getting this error on the line num = (num - temp) / 10, characters 25-27 : Error: Parse error: "end" expected after [sequence] (in [expr]). If I remove all of the begin end then I get the error Error: This expression has type bool but an expression was expected of type int at the same line.

let rec reverse_int num =
  if num / 10 == 0 then begin
    num
  end else begin
    let temp = num mod 10 in
    num = (num - temp) / 10

    let numDigits = string_of_int num

    temp * (10 * String.length(numDigits)) + rev_int num
  end;;