The Old Oak Doors are first mentioned in episode 15. John Peters — you know, the farmer — reported finding an old oak door suddenly standing unsupported by any other structure out in the scrublands. Sounds of knocking kept coming from the door, as if there were someone trying to get out from the other side. Uncertain which way the door opened, John Peters wrapped the door in deadbolts and chains on both sides. Cecil describes the door as "leading to other worlds than these", although it's unclear how he knows this.
John Peters says that he will keep a sleepless vigil over the door.
The old oak doors all seem to lead to the empty desert, which is inhabited by a great masked army, as well as several angels, all named Erika.
In episode 30, while trying to look for a way out of the dog park, intern Dana comes upon another old oak door, again unsupported by any other structure. She enters the door and finds herself in the House That Doesn't Exist, but in a parallel dimension. She exits the house through yet another door and finds herself in the vast and empty desert, looking at a mountain with a blinking light on the top of it, but it's unclear if the door she left through was another old oak door.
In episode 46, numerous additional old oak doors appear around town. Unlike the earlier doors, these doors are one-sided and cannot be seen from behind.