Contrary to what might be expected, though the order is not specified, in the expression e1 #m e2, native-code compilation evaluates e1 before e2 but bytecode evaluates e2 before e1. For example with

let () = (print_string "A"; object method m () = () end)#m (print_string "B")
class c () = object method m () = () end
let () = (new c (print_string "C"))#m (print_string "D")

ocamlopt prints ABCD, ocamlc prints BADC, with versions 4.02 to 5.2 on my machine.