In 2023, he joined the cast of Star Wars: Young Jedi Adventures as Jedi Master Estala Maru.
O'Brien is a prolific voice actor with over 400 credits across various series. He is known for often voicing villains and has provided voices for the English-language versions of a number of popular anime series. Some of his other notable animated roles have included a recurring role as Red Skull in Marvel's Avengers Assemble, a regular role as Gaara from Naruto, and Jushiro Ukitake in Bleach. O'Brien has also done voice work for many video games, including voicing the main villains in the games Final Fantasy XIII-2 and Eternal Sonata, Caius Ballad and Count Waltz respectively. He performed in World of Warcraft as Illidan Stormrage. He is also a member of Critical Role alongside Travis Willingham, Laura Bailey, Sam Riegel, and Matthew Mercer.
- Star Wars Rebels (2014—2018)
- Star Wars: The Old Republic: Shadow of Revan
- Star Wars Jedi: Fallen Order
- Star Wars: The Bad Batch (2021—)
- Star Wars Jedi: Survivor
- Star Wars: Young Jedi Adventures — "Stuck in the Muck"