Is blood type O RH positive compatible with O positive?

Yes and no.

When a blood type is described as being O+ or O-, there are actually two blood groups being described here: The letter (A, B, AB or O) refers to your blood type in the ABO blood group. The positive or negative sign refers to the Rh group; positive means that your blood cells have the D antigen, and negative means that your blood cells do not have the D antigen.

In the ABO system, the possible antigens a person can have are A, and/or B. O simply denotes the absence of A and B. In this system, the body's immune system naturally produces antibodies against the antigens that it lacks; so, an O individual will have Anti-A and Anti-B, an A individual will only have Anti-B.

In the Rh system, the immune system of an Rh-negative person generally does not produce Anti-D unless the person has been exposed to the antigen before through a previous transfusion or through pregnancy & childbirth. Once an Rh negative person is exposed to Rh-positive blood, there is an 85% chance that Anti-D will be produced.

When an Rh-negative person with Anti-D in their blood is exposed to the D antigen on Rh-positive blood again, they may have a transfusion reaction. Their immune system will mount an attack against the cells, and the result could be anywhere from a fever and hives up to shock and death. Luckily, the chances of death from a transfusion now are very small, and if the doctors notice a transfusion reaction starting to occur, they can often stop the transfusion before the reaction gets worse.

Many hospitals try to keep O-negative blood on hand for emergencies, but not every hospital is able to do this and O-positive may be the default blood for emergencies. The hospitals that do have O-negative blood on hand may only have limited resources; they may allow a patient to receive a limited amount of negative blood and if the patient is still bleeding, the medical director may allow the patient to be switched to positive blood. If an Rh-negative patient receives Rh-positive blood, they will give the patient an injection of Anti-D antibodies (such as WinRho or RhoGam) to attach to the D antigens, to reduce the chances of the patient's immune system noticing the D antigens and making it's own antibodies to it.

WinRho is also given to pregnant women who are Rh-negative so that if their fetus is Rh-positive, their immune system doesn't start to attack the fetus.

Anyhow, long story short: O positive isn't technically compatible and normally probably should not be given to O negative individuals, but in case of emergency, it is often better to give the O positive blood than it is to let the patient bleed out. However, O-negative people are normally compatible to give blood to O-positive people (unless there are other antibodies present in other blood group systems).