After a mate mated with a female, they would solemnize their union with the oath and pledge that they were one forever. The phrase "tlhIngan jIH!" ("I am a Klingon!") was part of the oath.
In 2365, Worf attempted to take part in the oath with K'Ehleyr, who in turn refused to take the oath. She later explained that her refusal of the oath was due to her being scared of the strong feelings she had for Worf. (TNG: "The Emissary")