A lot of 70's shows featured both TOS actors after Trek went off the air as well as future Trek spin-off actors in early roles.
The Six Million Dollar Man is one of the best examples having no fewer than 17 Trek guest stars:
William Shatner
George Takei
Marc Alaimo (Dukat)
Alene Martel (T'Pring)
Barbara Anderson (Lenore Karidan)
Bibi Besch (Carol Marcus)
Paul Carr (Lee Kelso)
Ted Cassidy (Ruk)
John Colicos (Kor)
Gary Collins (Gary Seven)
Yvonne Craig (Marta)
John de Lancie (Q)
Paul Fix (Dr. Piper)
France Nuyen (Elaan)
Henry Darrow (Admiral Savar)
Gary Lockwood (Gary Mitchell)
James Sikking (Styles)
The Six Million Dollar Man is one of the best examples having no fewer than 17 Trek guest stars:
William Shatner
George Takei
Marc Alaimo (Dukat)
Alene Martel (T'Pring)
Barbara Anderson (Lenore Karidan)
Bibi Besch (Carol Marcus)
Paul Carr (Lee Kelso)
Ted Cassidy (Ruk)
John Colicos (Kor)
Gary Collins (Gary Seven)
Yvonne Craig (Marta)
John de Lancie (Q)
Paul Fix (Dr. Piper)
France Nuyen (Elaan)
Henry Darrow (Admiral Savar)
Gary Lockwood (Gary Mitchell)
James Sikking (Styles)