Spoilers ahead for the The OA Season 2. With the introduction of the "house on Nob Hill" in The OA, Season 2 has gone borderline horror movie. The spooky home has captured the attention of a lot of people in the new dimension that Prairie travels to — including her alter-ego, Nina Azarova. And based on Karim and Prairie's journey inside, it's holding a lot of secrets that everyone wants to get their hands on.
Karim, a detective investigating the disappearance of a teenager named Michelle, first finds the house while on the case. Michelle is one in a large group of teens addicted to playing a game called Q Symphony, and the house is connected to the game — it is even, where the final level appears to lie. The first time Karim enters the house, he and his young guide find it empty... until another teenager/gamer emerges and attacks Karim like a rabid animal.
"My brain can hold all the brains," he says. "My thoughts can dry water. I've seen a million versions of myself." Sure sounds like he's encountered other dimensions in this house, don't you think? Unfortunately, he then jumps out a window to his death, leaving all questions unanswered. Later, when Karim consults security footage, he can't figure out when or how this kid even entered the house.
In Episode 2, Karim learns that the Nob Hill house is connected to a dream study in which participants all keep having dreams with the same three images: a tunnel the size of a coffin, a curved double-sided staircase, and a rose stained glass window. The only place in the world that has all three is the house on Nob Hill, and it turns out that eccentric tech guru Pierre Ruskin is using Q Symphony to lure players there in the hopes that they can unlock the secrets within.
"I don't know what that house is or what it does," says Ruskin's former associate Dr. Marlow Rhodes. "I don't think anyone does."
Much later on, after OA and Karim start to really investigate the house and piece together the mystery, Homer comes across an interview with Ruskin that explains its origins. This is his story:
The house was built in 1910, after the earthquake, by this childless couple in their '40s. He was an engineer, and his wife was a medium. They bought the land cheap. It had been the site of this massive mansion that had burned to the ground after the fires that followed the earthquake. The medium sensed that there was something strange about this site, and she was right. As they were preparing to lay the foundation for the new house, they discovered this natural spring. It turns out, this spring used to be the holy site of the Ohlone tribe, and the waters were said to give the shamans a God's eye view. The medium felt that they should not build on this site, but the engineer disagreed. He thought that since they had discovered the spring it was their duty to protect it.
In order to do so, they designed a house that is itself a puzzle. Only those who are worthy are able to solve that puzzle, and those who are not get "trapped and destroyed," according to Ruskin.
So while the house was designed by humans, the universe within it is, quite literally, of another world. Ruskin later tells Karim that the participants in his dream study also dreamed a fourth image, that of a man, and when they described him to sketch artists, the pictures all pointed back to Karim. He says that all this time, the house has been calling to Karim and only Karim.
The detective is initially skeptical, and dismisses the whole experience as a hallucination from the poisonous gases that emanate around the house. However, something pushes him to finish his investigation, and he goes back into the house. There, he makes his way to the same rose, stained glass window Michelle found, he's able to find and retrieve her from another dimension, and everything seems to be resolved — at least for Michelle and her grandmother.
As far as what the house actually is, it seems to be a portal to other dimensions. Similar to the robots or the tree garden Hap was growing in his secret room, it's just another way to travel. Granted, the way to get there seems a lot more convoluted (and dangerous) than the five movements, but we'll surely learn more about the house's significance if and when The OA's story continues to unfold. For now, it seems like the show is looking ahead to its next chapter — and dimension.