It's Hide. Try putting two and two together.>The OEG is not in Tokyo Ghoul>"Nagaraja", the king of snakes>"The ghoul closest to you">shapeshifting and immortal ghouls more or less confirmed a possibility>A mysterious voice that knows Touka, but she doesn't remember, tells her to seek refuge when the Oggai invaded>a few chapters before Hide was shown in the manga
Hide appeared after Kaneki lost his mother and befriended him.
Hide lead the CCG to Aogiri's HQ in part 1.
Hide appeared before Kaneki went to fight Arima and essentially saved him.
Hide gave Marude hints about the Washus and with Eto's book Marude was finally able to figure things out on his own.
Scarecrow has appeared every time someone close to Kaneki was in serious trouble.
Hide is the OEG and secretly also Kaneki's ancestor. He sought to protect his offspring in part 1 and has been pulling strings behind the scenes in part 2 for things to play out in Kaneki's favor despite all the curve balls that are being thrown at him.