I've had an idea. The Owl is the sacred Animal of Athena, the Goddess of Wisdom and War. This might seem irrelevant, but the fact that Bungie has used Ancient Greek mythological figures (Minotaurs, Hydras, Cyclopes) and even the weapon of Athena and Zeus, Aegis (VoG relic) lead me to believe that OWL is a threat assessment council / Early warning collective that functions as something in the middle between the convert reconnaissance of the Hidden and the overarching command of the Vanguard. Of course, there's no evidence, just speculation, but that's just my two cents on it.