they’re looking for typo-O in particular

They’re always looking for type-O, because O is the most able to donate. O- especially, which is the universal donor, though even O+ is great because it can donate to everyone with any positive type, which IIRC is like 80% of people. Other types are also important to donate because it lets them use the more specific blood type when possible, leaving the O-type blood for emergencies where determining blood type isn’t feasible.