Seizing on the idea of an acting career while in high school, David O'Hara left Scotland to pursue the craft at the age of 17, studying at London's Central School of Speech and Drama. The dark-haired actor made his feature debut in a small part in Bill Forsyth's Comfort and Joy (1984) and played a Scottish boy in The Monocled Mutineer (BBC, 1986), his first effort for the small screen.
Among David's stage performances, he portrayed Tybalt in a 1986 open air production of Romeo and Juliet and three years later took his turn in the play's more challenging role of Mercutio. His big break came as Stephen, the rampaging Irishman who joins forces with William Wallace (Mel Gibson), in Braveheart (1995). Two years later, he was co-starring opposite Helen Mirren as a slightly independent policeman in Granada Television's Prime Suspect V: Errors in Judgment which aired on PBS' Masterpiece Theatre. That same year (1997), David was very busy appearing in Alan J. Pakula's The Devil's Own; he was the romantic foil to Janeane Garofalo in The Matchmaker; he portrayed a biker in the Scottish film The Slab Boys and finally, he also portrayed Bill Sykes in /i>The Wonderful World of Disney adaptation of Oliver Twist (ABC).