The O5 Council originally had five members. They included the head members of four paranormal organizations from around the world (I'm fond of the idea that the Foundation was born from the alliance of preexisting groups), and one Administrator, the head of the Foundation, who acted as the tiebreaker vote. The title of "Administrator" was retired after the death of the founder, and all O5s have equal authority.
Following a deadly incident, the number of O5s was increased (though the name stuck), and the current rules against exposing O5s to anomalies were put into place. O5s are not permitted to interact with any anomalies under any circumstances; even if anomalies could save their lives. This means no anomalous life-extensions - every O5 dies of old age, disease, or mundane accidents, just like the rest of us. For all the power they hold, in some ways, they're more restricted than your average researcher.
O5s are typically groomed from birth for their role, and most of them are direct descendants of former O5s. This ensures that none of them are exposed to anomalies before they become O5s, and means that knowledge of the inner workings of the Foundation comes as naturally to them as simple addition.