The word 'Oh' has no real meaning, although by default it would seem to indicate either surprise or acceptance. No, like many short words of the English language (hmm and unh spring to mind), the word 'Oh' serves no purpose but to convey emotion through the tone of the voice that's saying it. For example, if I stressed the second half of the word ("oH!"), I could perhaps be pleasantly surprised ("oH! I love this toque!), perhaps be trying to call something to attention ("oH! Look out for that mini van!"). Or, if I said it completely monotone ("oh"), I'd most likely be resentful yet submissive ("oh. I see. Fine, I hate you"). I could go on and on, but 'Oh' can mean just about anything, if coupled with the proper voice and/or facial expression. It's quite a word, that 'Oh' is.