
Mark O'Brien (born May 7th, 1984) is a Canadian actor and director. He is best known for playing the role of Des Courtney in Republic of Doyle (2010) and Tom Rendon in Halt and Catch Fire (2014).

Arrival
2016 · as Captain Marks

Ready or Not
2019 · as Alex

Nuremberg
2025 · as Colonel John Amen

How It Ends
2018 · as Jeremiah

The Darkest Minds
2018 · as Rob Meadows

Marriage Story
2019 · as Carter Mitchum

Bad Times at the El Royale
2018 · as Larsen Rogers

Anon
2018 · as Cyrus Frear

Kin
2018 · as Jake

The Front Runner
2018 · as Billy Shore

Shadow of God
2025 · as Mason Harper

The Voices of Our Mother
2026 · as William

Seven Veils
2025 · as Paul

Parallel
2018 · as Josh

Blue Bayou
2021 · as Ace

The Righteous
2021 · as Aaron Smith

Little One

How to Plan an Orgy in a Small Town
2015 · as Bruce Buck