O positive (O+) is the most common blood type, carried by roughly 40% of the population in many countries. It means your red blood cells carry neither A nor B surface markers but do carry the Rh protein, which is what makes it “positive.” That combination of traits gives O+ blood a unique role in transfusions, genetics, and even certain health outcomes.
What Makes Blood Type O Positive
Your blood type is determined by two separate systems working together. The first is the ABO system, which looks at specific protein markers (antigens) on the surface of your red blood cells. Type O blood has neither the A antigen nor the B antigen. The second system is the Rh factor, a separate protein that is either present or absent. If it’s present, you’re Rh positive. O+ blood, then, is the combination of no A or B antigens with a positive Rh factor.
Your plasma tells the opposite story. Because your red blood cells lack A and B antigens, your immune system produces antibodies against both. This is why O+ plasma can only be given to other type O recipients, since the antibodies in it would attack A or B cells in anyone else. The rules for plasma donation are essentially the reverse of red blood cell donation.
How O+ Is Inherited
Blood type follows straightforward genetic rules. You inherit one copy of the ABO gene from each parent. The A and B versions of the gene are codominant, meaning if you get one of each, you’ll express both (type AB). The O version is recessive. To end up with type O blood, you need to inherit the O version from both parents.
That doesn’t mean both parents must be type O themselves. A parent with type A blood can carry a hidden O gene, and the same is true for a parent with type B blood. Two type A parents, two type B parents, or one of each can all potentially have a type O child, as long as both pass along their recessive O gene. The Rh positive trait is determined by a completely separate gene, and since Rh positive is dominant, only one parent needs to contribute it for the child to be Rh positive.
Who O+ Blood Can Help
O+ red blood cells are compatible with any blood type that is Rh positive. That means people with A+, B+, AB+, and O+ blood can all receive O+ red blood cells. This covers the majority of the population, which is why O+ donations are so valuable and so frequently used in hospitals.
The limitation is that Rh factor. O+ blood cannot safely go to Rh-negative recipients because their immune system may attack the Rh protein. This is what separates O+ from O negative, which is the true universal red blood cell donor. In emergency rooms, when there’s no time to check a patient’s blood type, the American Red Cross recommends starting with O+ red blood cells for males and for females who are beyond childbearing age. O negative is reserved for women of childbearing potential, where Rh sensitization could affect a future pregnancy.
If you’re O+ yourself, your options for receiving blood are more limited. You can only receive red blood cells from O+ or O- donors.
Supply and Demand Challenges
Because O+ is the most common blood type and compatible with all Rh-positive recipients, hospitals use it at a very high rate. The United States faces chronic shortages of type O red blood cells. O+ supply levels tend to fluctuate with seasonal peaks, sometimes approaching adequate inventory before dipping back down during high-demand periods. Blood banks actively encourage O+ donors to give regularly because the supply rarely keeps pace with need.
Heart Health and Disease Risk
A growing body of research links blood type to certain health risks, and type O appears to carry some cardiovascular advantages. A large meta-analysis found that non-O blood types (A, B, and AB) are an independent risk factor for coronary artery disease and heart attacks. The reason likely involves differences in clotting factors and inflammation levels. People with non-O blood tend to have higher levels of certain clotting proteins, which can contribute to blockages in the heart’s arteries.
One study published in Clinical and Applied Thrombosis/Hemostasis found a more specific benefit. When the heart’s main arteries do become blocked, the body can sometimes grow smaller backup blood vessels to reroute blood flow around the blockage, a process called collateral circulation. Researchers found that people with type O blood were significantly more likely to develop good collateral circulation compared to people with other blood types. In their study, 37.9% of patients with good backup blood flow had type O, compared to just 17.1% in the group with poor collateral development. This protective mechanism means that even when O-type individuals do experience a cardiac event, they may have better outcomes because those extra vessels help deliver oxygen-rich blood to the heart muscle.
On the flip side, some research has linked type O blood to a higher risk of stomach ulcers, likely because the surface chemistry of type O cells makes it easier for certain bacteria to attach to the stomach lining.
Pregnancy and Rh Factor
If you’re O+ and pregnant, the Rh side of your blood type is working in your favor. Rh incompatibility, a condition where a mother’s immune system attacks her baby’s blood cells, only occurs when the mother is Rh negative and the baby is Rh positive. Since O+ means you’re already Rh positive, this particular complication does not apply to you. There’s no need for the preventive Rh antibody treatment that Rh-negative mothers receive during pregnancy.
The ABO component can occasionally cause mild issues. If a mother has type O blood and the baby has type A or B (inherited from the father), the mother’s anti-A or anti-B antibodies can sometimes cross the placenta. This can lead to mild newborn jaundice in some cases, but it’s typically manageable and far less serious than Rh incompatibility.
O+ Compared to Other Blood Types
- O+ vs. O-: Both lack A and B antigens, but O- also lacks the Rh protein. This makes O- the universal donor for red blood cells, while O+ can only go to Rh-positive recipients. O+ is far more common (about 40% of the population vs. 9% for O-).
- O+ vs. A+: A+ is the second most common type at around 31%. A+ red blood cells carry the A antigen, so they can’t be given to type B or type O recipients. O+ has broader compatibility as a donor.
- O+ vs. AB+: AB+ is the universal recipient for red blood cells, meaning AB+ patients can receive from any blood type. But AB+ is rare, found in only about 2% of the population. AB is also the universal plasma donor, while O+ plasma has limited compatibility.