In the original The Omen (1976), Father Brennan said he was present during Damien's birth, that he was born in a hospital, and Brennan is said to have a 666 mark on his thigh suggesting he is also part of the cult. In this film, none of those things seem to be true. He appears to be trying to bring down the cult and does not appear to be present when Damien is born, nor did Damien appear to be born in a hospital.
In the original The Omen (1976), Father Brennan visited diplomat Robert Thorn in his office, started to argue, and shouted to him that his son's mother was a jackal. But in this movie, Father Brennan obviously knows his mother was American novice nun Margaret Daino. In the original The Omen (1976), on the gravestone of Damien's mother, it is written Maria Scianna, but in this movie her name is Margaret, and she didn't even recognize this surname.
The film takes place in June 1971; however, in the original The Omen (1976), the birth of the child took place one year earlier, in 1970. This is established when Ambassador Thorn and Jennings open the tomb of Maria Scianna and her child: the date of death on the tombstone is VI VI MCMLXX (6/6/1970 in Roman numerals).
(at around 1h 40 mins) The Latin in the birth scene is inaccurate - "Femina" is Latin for woman, the correct word for girl is "Puella".
(at around 46 mins) When Margaret goes to Father Brennan's apartment, he must unlock a well-bolted door. The top lock (a door chain) is fitted incorrectly allowing the chain to easily slip out the hole.
(at around 28 mins) The song "Daddy Cool" by the group Boney M. is heard in the disco scene. This song was released in 1976 and the film is set in 1971.
(at around 28 mins) During the disco/bar scene, the second song heard playing is "Domani" by Raffaella Carrà. The song was not released until 1980, but the film is set in 1971.
(at around 30 mins) During the disco scene, where Margaret goes to dance with Paulo, the song playing is "I've Got To Use My Imagination" by Gladys Knight & The Pips. The song was not released until 1973, but the film is set in 1971.
(at around 27 mins) When Margaret and Luz go to the disco together, the first song heard playing is "Rumore" by Italian singer Raffaella Carrà. The song was not released until 1974, though the film is set in 1971.
The fashion used by the supporting characters, as well as the extras in the film, do not belong to the early seventies, but to the middle of that decade.
(at around 48 mins) The explanation that a cult within the Catholic church wants to bring about the coming of the Antichrist in order to counter increasing secularism in the world is a flimsy one. Even if it was an attempt to restore the power the church once had over people by making them believe in God again, which Father Brennan says is the case. But, if they believed in their own teachings, bringing about the Antichrist would simply bring about the end of the world.
(at around 26 mins) It is hard to believe that a Catholic novitiate as devout as Margaret,vwhi has travelled halfway around the world just to take her vows, would ever go to a disco bar, especially dressed as she was, regardless of whether she had taken her vows yet.