This page is *theory* for Bloodborne, by Clone246
First, let's discuss what we already know about Oedon already and then expand on my take of the events in Bloodborne.
First of all Oedon is a formless Great One and only his voice exists in the waking world (we assume that Caryll, the only person who was able to hear and make runes based on Oedon's words, did NOT come in contact with dreams and nightmares created by the Great Ones.) We also know that his essence is blood and that he and his followers seek it and it is highly likely that Oedon is the Great One who impregnated Queen Yharnam and gave birth to Mergo.
Okay now here comes the speculation. Remember in Cainhurst when we come across Annalise? She is sitting on her throne but beside her is an empty throne, why is that? Queen Annalise keeps on saying "we" in her dialogue which implies that she is with the company of another being. With references to wanting a "child of blood" I can safely say that the being she is talking about is none other than Oedon. Annalise wants to give birth to a child of blood, Oedon's essence is blood, therefore Annalise wants to bear Oedon's child. Annalise gets offered blood dregs by her followers in order for her to have more of a chance to gain a child of blood, in a way Annalise is trying to appease Oedon by offering blood.
Moving on to the Tomb of Oedon, why would a Great One have a tomb dedicated to it? Well it is simple, Oedon in a way has died and lost his physical form, reducing his presence to a voice.This is further evidenced by the presence of Mergo, the formless baby Great One has no form, this is because it was a stillborn as indicated by the bloody clothes of Queen Yharnam. Now what is the deal with the Wet Nurse? Why would a dead baby need a Wet Nurse? To put it simply the Wet Nurse does what its job is but with a twist, instead of feeding Mergo breast milk it feeds Mergo blood. This is implied due to the husks of the members of the School of Mensis and the skeletal remains all around the area where you fight the Wet Nurse. I believe the Wet Nurse is feeding Mergo blood in order for Mergo to be reborn.
Oedon is most likely represented by the Moon, the Moon is ever present during the events of Bloodborne, it is shining through the window at Cainhurst's throne room, shining on the exact spot where Rom is in the lake, shining at Mergo's loft, present in the Hunter's Dream and finally shining on Ebrietas. Let's talk about Ebrietas, she is the daughter of the Cosmos, the exact same thing Micolash was trying to contact (Kos/Kosm) judging on the appearance of Ebrietas she looks exactly like a grown up version of the baby Great Ones all around the Choir's domain, those same baby Great Ones look exactly like the baby Arianna gave birth to when she got knocked up by Oedon. "The sky and the cosmos are one" I believe that this quote points to the Moon being present in the cosmos and in the sky which made me conclude that Kos/Kosm = The Moon = Oedon!
Lesser Amygdalas appear everywhere during the Paleblood moon sequence, I believe that these servants of Amygdala are there to spread fear and madness across Yharnam. The villagers that are struck by the scourge actually see YOU as a beast, they are afraid of you! They call you a foul beast and tell you to stay away as they wave their torches and pitchforks at you! This is all part of Amygdalas plan, to spread fear and madness in order for the Hunt to continue in the waking world, I believe this is all part of Oedon's plan but where does it all come to?
The answer is within the Hunters Dream, the only Great One known to reside in there is none other than the Moon Presence. I believe the Moon Presence is Oedon trying to be reborn again, manipulating the Doll and giving the Hunters a safe haven for one purpose, the Hunters kill beasts and gain blood echoes to exchange for power by the Doll, where does this blood go to? To Oedon himself to strengthen his physical form (starting to see a pattern here?) All the endless hunts, all the endless nights, all of Amygdalas' servants spreading fear and madness into all of the Yharnamites, it is all Oedon's plan to regain his physical form. Now there are arguments that the Moon Presence/Oedon is against other Great Ones and is using your Hunter to kill them, I disagree with this theory because the way I see it, the Great Ones are so much more powerful than humans, so why would they even bother relying on them to do their job? While I agree that they do not see humans as anything other than tools, whether it is using Gherman as a puppet to host the Hunter's Dream or whether it is impregnating women such as Yharnam and Arianna in order for their race to continue, I do not think that they would call for the aid of a human hitman. Plus if Gherman is indeed under control of the Moon Presence/Oedon, why doesn't he say to the player to hunt the Great One baby Mergo? It just doesn't add up. When the Moon Presence embraces your Hunter it isn't hugging you like a child, it is in fact draining you of your blood echoes so it can be one step closer into achieving its full physical form and come into the waking world, your Hunter was brought into the dream by blood, now that he is drained of it he is nothing but a shell for the Moon Presence/Oedon to manipulate. When your Hunter destroys the Moon Presence, Oedon isn't completely dead since Great Ones can never truly die (Amygdala is fear in physical form, when it is killed it doesn't mean we all stop being afraid) This is similar to Azathoth, both Oedon and Azathoth are in a dream like state and if they were to manifest in our world (wake up from their dream) it would destroy us all!