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