According to the novel, when Osha requested to serve Theon as fighter - he agreed immediately, on condition that she bent her knee and swore an oath to him, and she did. She did not offer to sleep with him, and there was never any sexual intercourse between them.
According to the novels, Sandor Clegane had been calling Sansa "Little Bird" since the Tourney of the Hand in the first book, when he begins to form a complex relationship with her by both mocking her courtesies and at the same time protecting her from Joffrey after Ned Stark is beheaded. In the show, the riot is the only time he says the nickname.
According to the novel, Petyr "Littlefinger" Baelish never set foot at Harrenhal, due to the alleged curse that destroyed every house which ever held the castle, and cost the life of nearly every person who ever commanded or garrisoned it, even for short time.
According to the novel, it was not Ser Rodrik Cassel but Farlen the kennel-master who was beheaded by Theon. Theon falsely accused Farlen of killing three of his men (who were actually killed by Ramsay Snow at Theon's command), and sentenced him to death. Neither Dagmer nor any of Theon's men talked him into killing Farlen.
According to the novel, it was not Farlen the kennel-master but Mikken the blacksmith who cursed Theon, when Bran announced that he yielded Winterfell. One of Theon's men killed him for his defiance by spear-thrust.
The episode title refers to the mutual oaths sworn by Brienne of Tarth and Catelyn Stark in the novel "A Clash of Kings". The scene was portrayed in the former episode Game of Thrones: The Ghost of Harrenhal (2012).
The trivia items below may give away important plot points.
According to the novel, Ser Rodrik Cassel was not captured by Theon's soldiers. After Winterfell was conquered in his absence, he returned with army and attempted to persuade Theon to surrender, but failed. Later he was treacherously killed by Ramsay Snow, Roose Bolton's bastard son.
According to the novel, Roose Bolton never suggested to Robb that his bastard son Ramsay would retake Winterfell. Ramsay was held a prisoner in Winterfell at that time, disguised as his servant Reek. After Theon captured Winterfell, he released Ramsay and accepted him to his service, unaware of Ramsay's true identity.
According to the novel, Amory Lorch was not one of the people whom Arya told Jaqen H'ghar to kill. After Roose Bolton and Vargo Hoat took over Harrenhal, they had him thrown into the bear pit, where he was torn apart by the bear.