In other countries, hand gestures mean very different things than they do in America. In Italy, the "OK" hand gesture to which you are referring does not have a positive meaning. "Worthless" is the closest direct translation, but it is a very crude gesture and certainly not one for polite company. Gestures like these can cause great consternation in international relations.
For instance, in some cultures, showing the soles of one's feet to someone else is very derogatory. However, in the Western world, men will generally cross their legs in such a way that reveals one's sole. There are many examples, even in high ranking political environments, when this faux pas has been committed. Additionally, in England, the "peace" hand sign, if shown reversed (with the back of the hand outward), is a very rude gesture.
There are many of these gestures throughout the world that get lost in translation.