When I've seen this in Spanish and Italian, it's usually something like "Aii!" (pronounced like "eye"). Oddly enough, when I lived in Italy, Spain and Colombia, I found that the more the local language dominated my mental space, the more likely I was to say "Ai" rather than "Ouch."

So perhaps it's not onomatopoeic, but simply relates to the sounds a language has easily available to it - ouch would be a bit difficult to spell in Spanish (perhaps "auch," but Spanish words never end that way), and very difficult to do in Italian (couldn't even guess - Bean, emanuela, anyone?).

And what would be an onomatopoeic interjection in this case? It would have to depend on the injury, I suppose...