The fourth season of the television series Buffy the Vampire Slayer premiered on October 5, 1999, on The WB and concluded its 22-episode season on May 23, 2000. It maintained its previous timeslot, airing Tuesdays at 8:00 pm ET. Beginning with this season, the character of Angel was given his own series, which aired on The WB following Buffy. Various Buffy characters made appearances in Angel, including Buffy herself; Cordelia Chase, formerly a regular in Buffy, and Wesley Wyndam-Pryce, who appeared in Buffy season three.

Daniel "Oz" Osbourne is a fictional character created by Joss Whedon for the television series Buffy the Vampire Slayer. The character is portrayed by Seth Green. Green also portrays the character in one episode of the spin-off series Angel.