A mate of mine got gonorrhea a few years ago as a result of OWO. He had been punting for over 20 years and that was the first time he had ever caught anything. As has already been said it is a question of weighing up the risks as against the pleasure to be obtained. Personally I always avoid OWO for that reason and that is my choice. However, I do sometimes kiss even though it is still possible to get herpes from kissing so at the end of the day it is down to each individual to make their own risk assessment.
There are lots of things in life that carry risks and it isn't possible to be 100% safe unless you totally abstain and I don't think that is a realistic option.