M'enfin !

Modified, thanks.

Added in tests. Thanks for your posts!

I can only test my solution... since nobody else tried the kata with OCaml:-) but this solution works without any error!
I don't have enough info from your post to know where the error is.
Nevertheless I think you need to use the following in your code (due to very very long ints):

open Num;;

Could you tell me if it works with this modification? If it works I will add it in the tests part.