iterative.ml

open Printf

let i = int_of_string Sys.argv.(1);;

let a = ref 0 in 
    for j = 1 to i do
        a := (!a + j)
    done;
    printf "%i\n" !a;;

recursive.ml

open Printf

let rec foo x a = 
    if x == 0 then a else foo (x -1) (a + x );;

let i = int_of_string Sys.argv.(1);;
let a = foo i 0;;
printf "%i\n" a;;

results

brianm@binky:~$ ocamlc -o recursive ./recursive.ml 
brianm@binky:~$ ocamlc -o iterative ./iterative.ml 
brianm@binky:~$ time ./recursive 10000000
143223616

real	0m0.227s
user	0m0.224s
sys	0m0.003s
brianm@binky:~$ time ./recursive 100000000
987459712

real	0m2.227s
user	0m2.221s
sys	0m0.005s
brianm@binky:~$ time ./iterative 100000000
987459712

real	0m2.346s
user	0m2.337s
sys	0m0.006s
brianm@binky:~$ time ./iterative 100000000
987459712

real	0m2.347s
user	0m2.336s
sys	0m0.006s
brianm@binky:~$