Dating based on blood type is apparently popular in Japan and South Korea. Check out what this page has to say about Type O blood types:

Type O: The Warrior Type-O people are outgoing, expressive, and passionate. They are highly motivated, natural leaders. Blessed with a strong physical presence, they aren’t afraid to gamble because they are so convinced they’ll win. Type O Blood people are said to set the mood for a group and to take on the role of creating harmony among its members. Their image is one of being peaceful and carefree. They are also thought to be big-hearted and benevolent, and they tend to spend money on others generously.They are natural athletes. They tend to be obsessive in their quest for success, and this can make them boring to others. Type O’s are outgoing, and very social. They are initiators, although they don’t always finish what they start. They appear to be levelheaded and trustworthy, but they often slip and make big blunders inadvertently. That is also the what makes O Types lovable. Creative and popular, they love to be the center of attention and appear very self confident. Type 0 personalities can be carefree, generous, independent, flexible, idealistic, goal-oriented, athletic, competitive, and adaptable. They can also be clumsy, flighty, jealous, greedy, unreliable, obsessive lover, vain loudmouths.

Romatic Compatability by Blood Groups: A is most compatible with A and AB B is most compatible with B and AB AB is most compatible with AB, B, A and O O is most compatible with O, and AB