The supernatural drama, based on a series of books by Gabriella Pierce, focuses on a young couple who agree to manage one of the oldest apartment buildings in New York City - only to begin to experience strange events.
Masohn and Buckley have now signed up to play a married couple living in the building, Deadline says.
Masohn will star as a photographer called Louise, while Buckley's character is a playwright called Brian.
The project is in second position for Masohn, which means that she will only join the cast if her current show, Fox's The Finder, is not renewed for another season.
As well as playing Isabel Zambada in The Finder, Masohn has previously appeared in shows such as Chuck, Castle and NCIS.
Meanwhile, Buckley played Clayton in One Tree Hill, which will come to an end after its current season. His other credits include parts in Privileged and Lipstick Jungle.
Rachael Taylor and Dave Annable have already signed up for the lead roles in 666 Park Avenue, while Lost's Terry O'Quinn will play the owner of the haunted building.