Actor James Mason acted as a sponsor to the production for actor Sam Neill. Mason originally suggested to the film's producers that they should check out Neill. Producer Harvey Bernhard had Neill flown in to London for an audition, paid for by Mason. Neill later reimbursed Mason for the airfare. Neill drew on some of Mason's mannerisms for his performance and characterization.
According to director Graham Baker on his audio-commentary, the camera jammed whilst shooting slate 666 [the same digit as the number of the beast which appears as a birthmark on the head of Damien Thorn], filming the scene where Barbara sees a vision of her dead burnt baby.
Even though the Nazarene's identity is never revealed in the film the book based on the screenplay of the final conflict added more details such as the child being born to a clan of gypsies which explains why he didn't have a birth record thus Damien couldn't find him.
In the scene where Damien is "praying" to Satan in his own private sanctuary, parts of his speech are taken directly from the novel "Là-Bas" ("Down There") by J K Huysmans (1891). In the novel the central character hears the lines whilst attending a Black Mass.
In Germany and Hungary, the film was released as "Barbara's Baby", in an effort to align it with Rosemary's Baby (1968). Reportedly, this title also appeared on some movie posters for the film in some countries prior to the issuance of an official title for the picture
The film's title when originally theatrically released in 1981 was "The Final Conflict" but the film has since generally become re-titled and now known as "Omen III: The Final Conflict" in order to for the movie to include the word "Omen" which exists as part of the title of all three other films in the franchise, The Omen (1976), Damien: Omen II (1978) and Omen IV: The Awakening (1991).
The film's storyline applied retroactive continuity changes to the franchise's time line. Damien Thorn had been a child in The Omen (1976) and a teenager in Damien: Omen II (1978) and could have not been an adult in his thirties by the time the film was released in present day times of 1981. As such, the movie significantly retconed the events from the first two films back further in the past to accommodate the series story's temporal shift.
Director Graham Baker says actor Rossano Brazzi would turn up to the set each day with the script under one arm and whistling the tune from the song he sang in the earlier classic musical South Pacific (1958).
The backwards 100 foot fall from the bridge was performed by stuntman Vic Armstrong who said of it for the 2005 book of 'Guinness World Records' that it was the most frightening stunt of his career. The majority of Armstrong's jump in the past had been less than 70 foot drops.
Since the Book of Revelations ends with Satan being defeated, Damien and his followers should be aware of this. That's why it makes no sense that they keep quoting from Revelations since it ends up spelling their demise (it does not predict that the Anti-Christ will destroy the earth as they keep claiming).
In this movie, just like the other movies in the Omen series, Damien doesn't directly kill anyone. He summons his followers to do it, or he summons accidents to happen, but he doesn't actually physically wound anyone. (Michael Myers, Freddy Krueger and Jason Vorhees are much more dangerous by comparison).
Overall the Omen series was very successful at the box office: Parts 1, 2 and 3 were all successful. This is compared to the Exorcist where part 1 and 3 were successful, but part 2 was a colossal bomb.
Jerry Goldsmith's score is missing the Ave Santani song this go around. It's also missing the Gregorian chant sound effects and the wonderfully weird and chilling Latin black mass whisperings which augmented the score in the first two movies.
This movie came out in 1981, whereas Exorcist 3 didn't come out till 1990. There was also another Omen movie, that came out just a few years later, Omen 4 The Awakening, because the Omen series has been more successful overall, wheras with The Exorcist series just the first film was successful.
The trivia items below may give away important plot points.
The movie's closing post-epilogue postscript, a quote from Chapter 21, Verse 4 from the Book Revelation read: "And God shall wipe away all tears from their eyes; and there shall be no more death, neither sorrow, nor crying, neither shall there be any more pain: for the former things are passed away. REVELATION XXI:4".