A:O is short for Ancient Ones. For those who aren’t familiar
with H.P. Lovecraft and similar authors, the Ancient Ones are
atrocious, diabolical, grotesque Gods who came from the universe
beyond the universe. Our dimension is just one of many.
Untold alienage exists just out of sight, but that doesn’t mean
hideous forces aren’t there. A:O also stands for alpha and omega
because the Great Old Ones influenced our genetics in the
very beginning and They are instrumental to man’s destiny. (Quote from Liber A:O - V. Satanas)