Two 100% virgins have oral sex. How likely is it that they could pass an STD to each other?

Very low, unless... Two virgins, if they have had no sexual contact at all, have a very low chance of carrying stds. However, there are 2 reasons not to count one this. 1) virginity is a subjective state (some "virgins" have had anal and/or oral sex). 2) some STDs can be transmitted via blood or mucosal membranes as well, so virginity is not a guarantee of being disease-free.