O+ (O positive) is a blood type. It means your red blood cells carry none of the A or B surface markers that define the other blood groups, but they do carry the Rh protein (that’s the “positive” part). It’s the most common blood type in the United States, carried by roughly 38% of the population.
How Blood Types Work
Your blood type is determined by two separate systems working together. The first is the ABO system, which looks at whether your red blood cells display A antigens, B antigens, both, or neither. If you’re type O, your cells display neither A nor B. The second system checks for a protein called the Rh factor (also called the D antigen). If you have it, you’re positive. If you don’t, you’re negative. Combine the two and you get your full blood type: O positive.
Because O positive blood lacks A and B antigens, your plasma naturally contains antibodies against both. This is important during transfusions: your immune system will attack red blood cells that carry A or B markers, which limits what blood you can safely receive.
Who O+ Can Donate To and Receive From
If you’re O positive, you can receive red blood cells only from O positive or O negative donors. Those are the only two types that lack both A and B antigens, so they won’t trigger your immune system.
When it comes to donating, O positive red blood cells are safe for anyone who is Rh positive, regardless of their ABO type. That includes A positive, B positive, AB positive, and O positive recipients. This broad compatibility makes O positive blood extremely valuable in hospitals. It cannot, however, go to Rh-negative patients, because the Rh protein on your cells could provoke an immune reaction in someone whose body doesn’t recognize it.
O Positive in Emergencies
When someone arrives at a trauma center bleeding heavily, there’s often no time to test their blood type. In those situations, hospitals reach for universally compatible blood. O negative is the first choice because it’s safe for virtually everyone. But O negative blood is relatively rare and always in short supply, so many emergency rooms have protocols to switch to O positive once the initial supply runs out.
This works because about 85% of trauma patients are Rh positive, meaning O positive blood is safe for the vast majority of people. Hospitals typically reserve their limited O negative supply for women of childbearing age, since giving Rh-positive blood to an Rh-negative woman could cause complications in future pregnancies. For everyone else in a life-threatening situation, O positive is the practical workhorse of emergency medicine.
How You Inherited It
Blood type is genetic. You carry two copies of the ABO gene, one from each parent. The A and B versions are co-dominant (both express if present), while O is recessive. To be type O, you need two copies of the O version, one from each parent. That means both of your parents carry at least one O gene, even if their own blood type is A, B, or O.
The Rh factor follows a similar pattern. Rh positive is dominant, so you only need one copy to be positive. You could carry one positive and one negative copy, or two positive copies. If both parents carry a hidden negative copy, there’s a chance their child could be Rh negative even though both parents are Rh positive.
So for a child to be O positive, both parents must carry at least one O allele, and at least one parent must be Rh positive. Two O-negative parents, for example, can only have O-negative children.
Pregnancy and Rh Status
If you’re O positive and pregnant, your Rh status actually works in your favor. The main blood-type concern during pregnancy is Rh incompatibility, which happens when an Rh-negative mother carries an Rh-positive baby. The mother’s immune system can develop antibodies that attack the baby’s red blood cells, potentially causing serious problems in that pregnancy or future ones. Because you’re Rh positive, this particular issue doesn’t apply to you. Rh incompatibility only affects Rh-negative mothers, who receive injections of Rh immune globulin during pregnancy to prevent the problem.
Why Hospitals Need O Positive Donors
O positive is the blood type hospitals use the most. Its compatibility with all Rh-positive patients, combined with its role as a backup in emergencies, means demand is constant. The American Red Cross specifically recruits O positive donors for a procedure called Power Red donation, which collects roughly twice the usual amount of red blood cells in a single visit. You can do this every 112 days, up to three times a year, though there are height, weight, and hemoglobin requirements that differ for men and women.
If you’re O positive and considering donating, your blood is in consistently high demand. A single Power Red donation can help more patients than a standard whole blood donation because it concentrates the component hospitals need most.
Health Associations With Type O Blood
Blood type has some links to disease susceptibility, though the effects are generally modest. One well-documented connection is cholera. Researchers at Washington University School of Medicine found that cholera toxin hyperactivates a signaling molecule in intestinal cells at roughly twice the level in people with type O compared to type A. The A and B antigens appear to act as decoys, drawing the toxin away from its real target, and the O antigen simply isn’t as effective at this. This helps explain why people with type O blood have been more likely to be hospitalized for cholera, something epidemiologists first noticed four decades ago.
On the other hand, some studies have found that type O blood is associated with a lower risk of certain cardiovascular problems, including blood clots and heart disease, compared to types A, B, and AB. The differences are real but small, and your lifestyle, diet, and family history matter far more than your blood type when it comes to overall health risk.