First, let me say that I really have no expertise in this diet or what is good for O blood type; I can only share what has worked good for me. I am O positive, and I did some research on which types of birth control pills reduce chance of weight gain. The kind I found is called Yasmin, and it has worked well for me. It is a low dose pill with a different type of hormone. I have found it does not cause weight gain like some other synthetic hormones do.