The Medium and the Engineer

After a harrowing ordeal, OA and Karim find themselves locked inside a labyrinth of puzzles. Nagging doubts prompt Homer to snoop on Dr. Percy.

Tropes present in this episode include:

Bizarrchitecture: The empty house appears to contain portals to other worlds.
Indian Burial Ground: It's revealed that the empty house was built over a spring that was considered sacred to a local Native American tribe. The medium sensed that nothing should be built there, but the engineer went ahead anyway.
The Reveal: Nina Azarova is a medium and could communicate with nature, which is what allows the OA to communicate with Old Night and the trees outside the empty house. They tell her that Hap will make a huge discovery that will convince her she needs him to survive, but she can protect herself by forming a tribe, and that Nina Azarova is the key to her survival.
Secret Underground Passage: The OA and Karim end up in one that leads to the mysterious empty house.
Title Drop: Homer finds a tape in Hap's office, in which someone describes the couple who built the empty house; the wife was a medium, the husband was an engineer.