The official synopsis of the third episode of Arrow reveals Onyx will be played by Chastity Dotson:
DC CHARACTER ONYX VISITS STAR CITY — A rogue black ops team led by Onyx (guest star Chastity Dotson) breaks into Kord Industries and steals something lethal. Oliver (Stephen Amell) struggles to connect with William (guest star Jack Moore) so he reaches out to a surprising source for help. Kevin Tancharoen directed the episode written by Speed Weed & Oscar Balderrama (#603). Original airdate 10/26/2017.