O'Brien's performance in The Train (1964), in which he played a Wehrmacht Feldwebel, led to his first break-out role in Grand Prix (1966) starring alongside James Garner and Eva Marie Saint.
In spite of this, O'Brien continued to work for another decade in the Italian film industry, almost exclusively for directors Lucio Fulci and Joe D'Amato.
His father then returned to Ireland with the pension he received for his military service, sold the family farm and retired to the South of France where he eventually met and married an English governess.
[3] Growing up, he was a great admirer of fellow Irishmen William Butler Yeats and Michael Collins, the French adventurer André Malraux, composer Maurice Ravel, the Italian artist Giorgio de Chirico, German boxer Max Schmeling, English actor Sir Laurence Olivier and especially handicapped Second World War ace Douglas Bader.
He received leading roles for several local stage performances and, after joining the Dublin Gate Theatre, was involved with productions headed by Irish dramatist Micheál Mac Liammóir.
[3] In 1953, the 23-year-old O'Brien made his first appearance in a feature film, Anatole Litvak's war drama Act of Love, in which he had a brief speaking role.
He spent the next few years in France and had minor roles in several other films including The Wretches (1960), Saint Tropez Blues (1961), Dynamite Jack (1961), Tales of Paris (1962) and, in an uncredited role, as an English priest in Robert Bresson's The Trial of Joan of Arc (1962); he also made his French television debut guest starring on L'inspecteur Leclerc enquête.
[3]His later Grand Prix co-star James Garner, however, struck him as, ..a very good-looking fellow, all the girls went crazy for him, and a good actor at that, but he, like others I used to work with, seemed to be self-conscious and nervous, for no discernible reason.
[7] His portrayal of ex-American lawman turned soldier of fortune Nathaniel Cassidy led to future leading roles in the genre for a number of years.
It was during his years working in Italy that he changed his given name from "Donal" to "Donald", given his film contracts and credits frequently misspelled his name, banks would refuse to cash his checks under his birth name.
[3] By the early 1970s, however, the genre was already starting its slow decline and saw O'Brien, usually a villain (or occasional anti-hero), in increasingly low-budget productions such as Giuseppe Vari's The Last Traitor (1971), with Maurice Poli and Dino Strano, Paid in Blood (1971) with Jeff Cameron and Sheriff of Rock Springs (1971) with Cosetta Greco and Richard Harrison.
He later recalled having a somewhat strained relationship with Berger, mostly due to his drug issues, and was given parts originally intended for the older actor when was either unable to perform or had been arrested.
O'Brien also starred in one of his first non-western roles, in the Italian horror film Il sesso della strega, as the investigating police inspector.
In the film, O'Brien played devout Mormon missionary Lester O'Hara, half-brother of Harrison's womanising amoral character Jesse Smith.
In the film, he and his wife Maggie, played by another one-time Spaghetti Western star Susan Scott, encounter Emanuelle (Laura Gemser) in the Amazon and join her expedition to find a lost tribe of cannibals.
Stricker in Gianfranco Parolini's Yeti: Giant of the 20th Century (1977), mercenary Major Hagerty in Joe D'Amato's Tough To Kill (1978) and as the SS Commander in Enzo G. Castellari's The Inglorious Bastards (1978).
The first was a cameo appearance in the sex comedy Sesso profondo and the second, a much larger role, in Zombie Holocaust as the main villain Dr.
In 1986, he played another "mad scientist" in Sergio Martino's science fiction film Vendetta dal futuro/ aka "Fists of Steel".
He was also supposed to appear in a somewhat risque cameo for Tinto Brass' erotic film Paprika but his scene was lost on the cutting room floor.