He met Crispin Freeman while working on a production of Romeo & Juliet in Cincinnati, Ohio, which helped him get voice-over work back in New York. After moving to New York City to attend the New York University Tisch School of the Arts, he worked in theater on various productions around the country. His family were Reformed Christians, and he was 'raised by a Christian mother and attended 15 years of Christian schooling', but would later describe himself as agnostic.

His mother, Lois Wiltse O'Brien, worked as an educator as well as a quality assurance trainer and consultant. Liam Christopher O'Brien was born in Weehawken, New Jersey, on May 28, 1976. He has also voice directed for video games such as The Last of Us, Evolve, Resident Evil 5, and Resident Evil 6. He is an automated dialog replacement writer for anime and has provided voice direction for over 300 episodes of anime such as Naruto. In video games, he voiced Gollum in Middle-earth: Shadow of Mordor, Caius Ballad in Final Fantasy XIII-2 and Lightning Returns: Final Fantasy XIII, War in Darksiders, Asura in Asura's Wrath, Ieyasu Tokugawa in Sengoku Basara: Samurai Heroes, Illidan Stormrage in World of Warcraft and Heroes of the Storm, Yasuo in League of Legends, Zvatto in Lost Planet 2, Grimoire Weiss in NieR and Nier Replicant ver.1.22474487139., the Warden in For Honor, and Barker in Titanfall 2. In cartoons, O'Brien has voiced characters in shows such as Star Wars Rebels, Transformers: Robots in Disguise, Avengers Assemble, Wolverine and the X-Men, and Hulk and the Agents of S.M.A.S.H. His major anime roles include Gaara in Naruto, Vincent Law in Ergo Proxy, Captain Jushiro Ukitake in Bleach, Lloyd in Code Geass, Kenzo Tenma in Monster, Akihiko Sanada in Persona 3, and Nephrite in the Viz Media dub of Sailor Moon.

He has been involved in many video games, cartoons, and English-language adaptations of Japanese anime. He is a regular cast member of the Dungeons & Dragons actual play series Critical Role, playing Vax'ildan ('Vax'), Caleb Widogast and Orym. Liam Christopher O'Brien (born May 28, 1976) is an American voice actor, writer, and director.

Fallout 4 ranger armor