The oath-taking is held in Odo's church at Bayeux; and his priests cause Harold to stand between two alters containing the sacred host. He swears to be your vassal for his lands in England, and any other place where he may hold from you. And he swears to uphold your claims to the English crown.
Then Odo has the draperies of the alters moved aside, revealing that they are in fact reliquaries, containing the earthly remains of holy Saints: Harold has sworn upon holy relics, and does not appreciate your tricking him. He does not say anything, but you can tell that he feels betrayed in his new friendship. Soon afterward, you let him return to England, reminding him of his sacred oaths to you. (It was a low trick, but he wasn't cooperating at all, was he?)
Within two years, making it early January 1066, you learn that king Edward is dead, and Harold Godwinson was crowned king on the very day of Edward's funeral.
That is the guantlet thrown down all righty. What will you do?
Send stern warnings to Harold, that you are going to invade if he does not stand down?
Support Tosti, Harold's exiled brother, in his effort to destroy him?
Work in with Harald Hardrada, king of Norway, who has a previous claim on the English throne?