Hall & Oates (or "Oates & Hall") are an American musical duo comprised by Daryl Hall, who mostly sings lead vocals and John Oates who primarily plays electric guitar and provides backing vocals. They appear as themselves during Grace's wedding in the episode The Definition of Marriage.
According to Karen Walker, he once did Jell-O shots with Oates during the 70s. Karen and Hall also mention having had a sexual encounter in Dayton, Ohio in 1981 with rock musicians Chrissie Hynde (The Pretenders) and Joan Jett (Joan Jett & the Blackhearts).
During Grace and James' wedding, Karen hires Hall & Oates to sing and serve drinks. By this time, they have changed their name to "Oates & Hall" as part of an agreement to "flip it" every 25 years. According to Hall, he did not think they would last that long when he agreed to it.