The first thing to note is that the O/B/A part is completely different from and independent of the +/-, but they both refer to immunological variability among the red blood cells. All your red blood cells are the same of course, but they are very likely to be different from mine, and less likely to be different from those of your sister [it's the genetics!]. Why immunological? Because the immune system is tasked to recognise foreign entities in blood and tissues and knock 'em on the head before they start a riot. You are made up of about 100 trillion cells, all with 46 chromosomes and 'human', but you also tote around about 200 trillion cells of maybe 10,000 different species of bacteria. We can't function at all at all unless both of these cohorts are present: they are recognised as 'self' or tolerated as 'commensal'. The cells of the immune system, mostly various types of white blood cell, patrol the body looking for things, mainly proteins, that are non-self. These include nematode worms, malarial parasites, Vibrio cholerae, and HIV. But also, amazingly and effectively, they recognise the rogue males of the body - cancer cells - and almost always nobble them before the incipient tumor is detectable by the most sensitive equipment in the Dana Farber Cancer Institute. Go immune system!
The way white cells approach the rest of the 100 trillion is to get up close and personal and frisk them for the proteins that are sticking out of the cell membrane. If these proteins are within the normal range, which the immune cells have been practicing on since birth, then they are left alone; if they feel a bit funny then that cell is immediately dismembered and the parts recycled. The sticky-out proteins are called antigens because they generate a response from the antibodies of the immune system.
Prev], we've called this system of antigens the ABO blood groups. A has one antigen; B has the other; AB have both; and O have neither. The frequency of these blood groups varies across the world [see above for the B group which is commonest in a belt across central Asia and completely absent among Native South Americans. In all populations, the gene frequency of the O variant is always above 50%, so is most common. We have theories but no certainty about why there is this pattern of variation. If Hercule Poirot addresses a multi-racial roomful of people and says that one person present matches the blood group B which was found on the bloody knife left in the bread-bin - put money on it being the lady from Pakistan.
Nobody has given me a convincing explanation of what these little proteins do in the normal body; or the even greater mystery about why we are all charged up to deal adversely with certain blood-transfusions.
But here's a further peculiarity about ABO blood groups, which might give us clues about why we're all different, Group O is more likely to develop squamous cell and basal cell carcinoma but less likely to get pancreatic cancer. So it's like the CCR5 mutation which resistance to HIV but makes the carriers more susceptible to West Nile Virus. There is some evidence that gastric cancer is more prevalent in Group A. These conditions are comparatively rare and were much rarer when we all died young of hyena, spear or pneumonia. There is a hint of evidence that Group O are more susceptible to cholera, which was a serious scythesman among any static conglomeration of people where the water supply could get contaminated with the runs.