Project Euler 10 - Summation of primes
Here are some comments and an example rewrite of your code.

Indentation and line breaks are not idiomatic to OCaml, it's good practice in any language.

A more intuitive type for is_prime would be to have only one argument, so let's encapsulate is_prime_rec:

let is_prime =
  let rec is_prime_rec number start =
    if start * start &gt; number then true (* OCaml provides a type `bool`, distinct from `int`, so it's better to return `true` instead of `1`. *)
    else if number mod start = 0 then false
    else is_prime_rec number (start+1) in
  fun n -&gt; is_prime_rec n 2;;


which yields val is_prime : int -&gt; bool = &lt;fun&gt;.

You know the number of iterations in the loop, so better use for rather than while.
This also avoid manually incrementing i.

let sum = ref 0 in
for i = 2 to 2_000_000 do (* detail: OCaml parses `2_000_000` as `2000000`, which is more readable. *)
  if is_prime i then (* no need for parentheses around the if clause *)
    begin
      sum:= !sum + i;
      Printf.printf "%d is prime, it is prime number %d\n" i !sum;
    end
done;;


I can't help but add an algorithmic remark: consider using a sieve.