My theory I believe has a lot of valid points, but please do give me feedback. I believe the One Eyed King is Hisashi Ogura. Lets look at the evidence here. Kaneki's state of mind could not be fully trusted at the time he recalled Nico telling him who the One Eyed King was, this is common fact among the community. Where Ogura fits into this is that he knows a lot about ghouls, and not only this but supports their rights as a 'human'. If you look into the small kanji whispered from Nico when he tells Kaneki who the OEK is, it can roughly translate to 'Ghoul lover'. Ogura wore a headband with this phrase etched onto it, and not only that, but I believe that Arima slit his throat out of cowardice, as Ogura can be seen ever so faintly in his glasses' reflection. It is so blatantly obvious and I felt it was quite plausible. Ogura is SSSS Rated at least and I believe we should share this as much as we can #OguraIsTheOEK