I'm O positive, which is almost as good as O negative. I can donate to any blood type, but only for RH-positive patients.
I've donated blood over 30 times, and I'm hoping to get to 50 someday. Platelets a few times too. Donating platelets takes longer, but leaves you "perkier" since you don't lose blood volume.
I try to schedule my blood donations every 2 months, because there's a 56-day minimum between donations.
I get all the juice and cookies I like afterwards. I usually drink 3 of those little juices, and have my traditional Fig Newtons.
My blood donations are for anybody who needs them, as opposed to directed donations for a particular person, but just last week I received notice that a casual acquaintance needed blood donors, so I called back to arrange it. Turns out he got so many offers he doesn't need me, at least not yet, so I'll be back at my regular donor center next week.
One time I showed up at the donor center and didn't realize it had been only 55 days since my last donation, not the minimum 56. I thought they would say "close enough", but they said they have to follow the rules. Then a nurse said "Hold on, I have an idea." She went away and came back with a doctor in tow, who told me that directed donations were permitted below the minimum intervals (intended for people helping relatives), so that I could donate it to a particular pediatric patient of his who needed a transfusion that day. I said sure. They keep donations anonymous, but I saw the paperwork upside-down from a distance, and could see that I was donating for a girl named Allison. It shouldn't matter who gets my blood, but it made me particular happy to know that I was helped Allison, whoever she is. I hope she's ok now.