O5-1 is the first member of the O5 Council, the ruling council of the SCP Foundation. As with other O5 members, O5-1 is an enigmatic figure, whose true identity and appearance are completely unknown to the rest of the Foundation, save for the rest of the Council and select individuals. In spite of this, there are several theories as to O5-1's identity and appearance, ranging from being a group of people to simply only existing as a concept.
However, it is consistently believed among outsiders that O5-1 is the most powerful member of the entire council; this is highly debated due to massive amounts of misinformation having been spread about the Councilors.