Ow is a sound commonly heard by one in severe pain, and is thus exhibiting such pain verbally. Sometimes, pain can be too much for the human to bear; thus it derives the involuntary sound, "Ow!" Other such sounds involving severe pain include "Ouch", "Ah", "Ooh", and sometimes, simply just a sharp, audible intake of breath that sounds oddly like hissing or an attempt to keep the mouth closed while involuntarily shrieking in pain, resulting in a strange-sounding humming grunt.
 In spanking
"Ow" is perhaps the most commonly heard sound during a spanking. The spankee will often react involuntarily to the pain brought on by the spanking, and display the reaction verbally in the form of that sound, or any of the aforementioned sounds. Other common sounds heard from the spankee during a spanking include "Oh" and "Ah", but the most often used is nearly always the only used.
These sounds are especially prevalent if the spankee has his/her buttocks exposed for the spanking. For example, a female spankee laying across the knee of a male spanker will often lift the spankee's skirt/pull down her pants, yank the spankee's panties down to reveal the uncovered backside, and proceed to spank her with his hand or with an implement, whereupon the spankee will automatically react to each painful smack, the more painful blows often forcing the most, or the highest-pitch, squeals, yelps, and uncontrolled "Ow"s and "Ouch"es from her vocabulary.
 "Ow" in other languages
It is interesting to note that exclamations such as "Ow" are culture and language dependent. For example, a French, Spanish or Portugese speaking person exclaims "I" (spelled Aïe, Ay or Ai) instead of "Ow". In South Scotland, North England and Hong Kong, they say "Ai-ah"; likewise in Italy where it's spelled "Aiya". A German speaker may yell "Ow-ah" (spelled Aua), "Ow" or "Ouch" (spelled Au or Autsch, respectively). A Russian says "Oy", an Arabic "Ayy", "Aw", or "Akh", an Indonesian or Malaysian "Aduh" or "Adoi", a Chinese "Ai-oh", a Japanese "Itai".
It seems that a child picks up such exclamations from their peers and family at a young age and then it becomes part of their "vocabulary" — although they wouldn't think of it as a "word" in the normal sense, and the exclamation seems to come "by itself" without much possibility for control.
Example of "Aïe!!" in a spanking drawing by erik (2000)