What do you think ? I think he is. One of the big clues, is that Ferrus Manus in one of the books saw a hallway filled with white statues, with each one representing a certain Primarch. Ferrus recognizes one of the statues as the two-faced roman god, Janus ( which is clearly meant to be Alpharius/Omegon ). Also, I remember reading somewhere that Omegon kept a hidden suit of grey knight armor with him.