To make the baboons attack the car in the Windsor Zoo park scene, an official from the zoo was in the back seat of the car with a baby baboon, but the baboons had no response at all. They then took the head of the baboons, and the baboons outside went crazy. Lee Remick's terror as the baboons attack the car was real.
Having changed its title from "The Antichrist" to "The Birthmark," the film seemed to fall victim to a sinister curse. Star Gregory Peck and screenwriter David Seltzer took separate planes to the UK...yet BOTH planes were struck by lightning. While producer Harvey Bernhard was in Rome, lightning just missed him. Rottweilers hired for the film attacked their trainers. A hotel at which director Richard Donner was staying got bombed by the IRA; he was also struck by a car. After Peck canceled another flight, to Israel, the plane he would have chartered crashed...killing all on board. On day one of the shoot, several principal members of the crew survived a head-on car crash. The jinx appeared to persist well into post-production... when special effects artist John Richardson was injured and his girlfriend beheaded in an accident on the set of A Bridge Too Far.
Richard Donner and Harvey Bernhard asked Alan Ladd Jr. then the head of Twentieth Century Fox for extra money during the film's post-production period to hire composer Jerry Goldsmith, whose music they strongly felt was right for the movie after seeing him perform a live concert at the Hollywood Bowl earlier that year. Ladd was finally talked into giving Donner and Bernhard around $25,000 to hire Goldsmith, who would deliver his first and only Academy Award win for his score in 1977. Donner credits the success of the film to Goldsmith's score which made the film scarier than it would have been without him.
As part of its pre-release publicity campaign, and to point out the significance of "the three sixes" as The Sign of Satan, the movie was sneak-previewed nationwide in the USA on 6 June 1976. While audiences inside the theatres were being scared witless by the film, theatre employees were out front, busily putting up specially made posters declaring: "Today is the SIXTH day of the SIXTH month of Nineteen-Seventy-SIX!" Hokey though it was, the gimmick worked quite well, as many a theatre patron literally "freaked-out" upon seeing those posters as they left the previews.
According to at least one biography of Gregory Peck, he took this role at a huge cut in salary (a mere $250,000) but was also guaranteed 10% of the film's box office gross. When it went on to gross more than $60 million in the U.S. alone, The Omen became the highest-paid performance of Peck's career.
Harvey Stephens, as Damien, was largely chosen for this role from the way he attacked Richard Donner during auditions. Donner asked all the little boys to "come at him" as if they were attacking Katherine Thorn during the church wedding scene. Stephens screamed and clawed at Donner's face, and kicked him in the groin during his act. Donner whipped the kid off him, ordered the kid's blond hair dyed black and cast him as Damien.
Because Harvey Stephens was so young, Richard Donner found that the best way to direct him was to provoke genuine reactions before the camera. For example, when Damien is angry at being taken to church, Donner got his peeved facial expression by shouting to Stephens off camera "What are you looking at you little bugger? I'll clobber you."
Billie Whitelaw was performing in a play at the West End when the film was being made. She rang up to ask if she could get out of making the film but as it was late on a Friday afternoon there was no one in the production office to receive her call. Whitelaw had no option but to show up for filming on the following Monday.
Supposedly David Seltzer's screenplay was inspired by a passage in Revelations about the destruction of the world and the rising of the devil. The Bible actually contains no such passage; it was simply a product of 20th Century Fox's publicity department.
One of Richard Donner's first requests to screenwriter David Seltzer was to remove all suggestions of the supernatural, such as cloven-hoofed demons and witches' covens. The golden rule was that nothing was allowed in the script that couldn't happen in real life. The idea was that there should be some degree of doubt over whether or not Thorn was deranged.
The biggest problem with shooting with Mrs Baylock's ominous dog was that the animal was nothing like the creature he was supposed to be portraying. He wanted to lick and play with his co-stars rather than threaten them.
The taxi driver who takes Thorn around Italy was the son of a Mafioso in real life and was very keen to get into the movies. In one scene, he can quite clearly be seen sporting a large bandage on his thumb - this was because Gregory Peck accidentally slammed a car door on his hand, nearly taking off his finger.
Gregory Peck had essentially been retired for seven years when he quite unexpectedly agreed to make the film. By doing so, he effectively validated the production and other members of the cast and crew soon came on board.
Richard Donner made a TV movie prior to this with Jack Palance called "Bronk". When Palance heard that Donner's next project was to be made in England, he urged his director to seek out his daughter as she was a great actress. Donner did precisely that and subsequently cast Holly Palance as Damien's ill-fated first nanny.
Mrs Baylock was originally written as a warm, effusive Irish nanny. For her audition, Billie Whitelaw significantly changed the dialogue to create the cold, slightly sinister character that was subsequently used in the finished film.
Jerry Goldsmith didn't want to attend the Academy Awards that year as he had already lost multiple times before and didn't want to go through the ordeal of losing again. The Omen was Goldsmith's only Oscar.
When 'Alan Ladd Jr' agreed to come on board as the film's producer, he insisted on Richard Donner joining the production as director. Donner - up to then, mainly a TV director - leapt at the chance to direct a major feature.
Richard Donner had read the script in one sitting and had heard that 20th Century Fox had already passed on the project. He repitched it to studio head Alan Ladd Jr. when the two were sitting down to dinner one night. Ladd was impressed with his enthusiasm and committed to the project.
In the closing scene, Richard Donner used reverse psychology on young Harvey Stephens telling him, "Don't you dare laugh. If you laugh, I won't be your friend." Naturally, Stephens wanted to laugh, and he instead smiled directly into the camera.
It was Richard Donner's intention to film and edit the scene where Jennings is decapitated in such a way that the audience, having closed their eyes at the beginning of the scene, would open them only to see the head still floating in the air.
The original ending had all three members of the Thorn family killed at the conclusion. Studio head 'Alan Ladd Jr' felt that this was a mistake. After all, the devil is impossible to kill, he reasoned. He subsequently gave Richard Donner the additional funds necessary to shoot a more open-ending closing to the film.