Granted, there’s a certain amount of onematapaea (sp?) involved, but why not “ieee” or “ack” as the word most associated with physical injury?
Much better people will be by shortly to answer your question, but you have caused a 3x5 index card in the back of my head to pop up.
“Ouch” makes a one-second appearance in the Story of English series by Robin McNeal. Check the section on German origins of (now) English words.
“Ouch” isn’t the universal word for pain:
Ouch Around the World
http://www.diseaseworld.com/ouch/ouch.htm
My favorites are Hungary and the Philippines.