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Oz appears in the first season episode "In the Dark", crossing over from Buffy to give Angel the Gem of Amarra, which will grant him virtual immortality, thus eradicating any vampire weaknesses, such as sunlight or being killed via wooden stake. The Gem is stolen by Spike, also crossing over from Buffy and Oz assists the Angel crew in their search for him. Oz leaves Los Angeles again after the adventure is over, and Angel decides to destroy the Gem after he has taken it back from Spike.
Buffy appears in two episodes of Angel's first season, first in "I Will Remember You", where she and Angel rekindle their love after he becomes human as a result of a Mohra demon's blood. After Angel learns from The Oracles, conduits to The Powers That Be that Buffy will die if he remains human, he asks them to turn back time, and prevents himself from being effected by the Mohra blood. She returns in "Sanctuary" to oppose Faith, leading to a conflict between Angel that was resolved in the following crossover, in the Buffy episode "The Yoko Factor". Buffy was at first thought to have appeared in the fifth season episode "The Girl in Question", but Sarah Michelle Gellar did not appear in the role, and in the Buffy Season Eight comics it was also revealed to be a decoy and not the real Buffy.
Faith first made the jump to Angel in its first season, in the episode "Five by Five", hired to kill Angel. After kidnapping and torturing Wesley, she engages Angel in combat, secretly acting on a death wish. Angel is able to convince Faith to walk the path of redemption, the plot of the following episode "Sanctuary". Thanks to Angel's faith in her, Faith agrees to turn herself in and go to prison for her crimes, and Angel occasionally visits her, as he does in "Judgement". Faith returns in the show's fourth season, in the episode "Salvage", broken out of jail by Wesley in order to hunt down and bring in Angelus. Almost at the cost of her own life, they (with some assistance by Willow) succeed to re-ensoul Angel, and Faith goes with Willow to appear in Buffy's seventh season.
Willow first appeared on the phone in the season two episode "Disharmony", calling with Cordelia about Harmony. She again appeared, but in a non-speaking role, at the end of the second season finale "There's No Place Like Plrtz Glrb", to tell of Buffy's death in the "The Gift", over on Buffy. Willow's last appearance is in the fourth season episode "Orpheus" to re-ensoul Angel as she did in the second season of Buffy. During this short time with the team, she connected with the members of Angel's crew, notably Wesley (in fact, Alexis Denisof, the actor portraying Wesley, is married to Hannigan in real-life), who shared their respective experiences with the darkness inside them. As an extra to the adventure, Willow was able to take Faith back to Sunnydale to aid in the ongoing events of Buffy's seventh season.
The last of the Buffy alumni, Andrew appears twice in the show's fifth and final season. First in the episode "Damage" as a representative for Buffy's gang to pick up new rogue slayer Dana. It is Andrew that expresses the group's new distrust for Angel's team, reuniting with the resurrected Spike at the same time. Andrew ultimately tricks Angel's team into giving up Dana after he appears with a large group of slayers. Andrew returns in "The Girl in Question", as a support to the Buffy decoy (as revealed in the Buffy Season Eight comics), giving Spike and Angel some advice on life and telling them it's time to let Buffy move on (for now).
