This episode explores Walter and Elizabeth Bishop's attempts to return Peter to the parallel universe using the Cortexiphan-induced abilities of young Olivia Dunham, while Walternate in the parallel universe struggles to deal with the kidnapping of his son.
This episode's glyphs spell:
Young Olivia inadvertently switching universes when trying to find Walter is what finally revealed to Walternate where his son Peter had been taken.
The Observer can be seen in the lobby of Bishop Dynamic.
The "Tulip Food Store" signage in '6B' foreshadows Olivia and Peter meeting for the first time in a field of tulips in 'Subject 13'.
The periodic table of elements in the Jacksonville daycare center in 'Subject 13' displays the element Osmium, the element featured in 'Os'.
These innate skills are what the Cortexiphan drug trials attempted to bring forth in children in "Subject 13".
Olivia didn't remember being in the trials as a child in 'Subject 13'.
Walter and Elizabeth tried very hard to gaslight Peter and make him forget and disregard any thoughts he had about his kidnapping and his real home in "Subject 13".
We learn in the very next scene that Walternate ignored his wife's requests to repair their marriage. In "Immortality", many years later, Walternate had a Japanese mistress named Reiko.
A radio broadcast from the Redverse in "6:02 AM EST" mentioned the Dodgers playing at Ebbets field in Brooklyn.