While Kathy is watering the plants on the estate's second floor, Damien rams her with his tricycle, causing her to fall over the balcony. The injuries Kathy sustains from the fall cause her to miscarry.

Robert calls Kathy in the hospital and tells her he's sending Tom Portman to bring her to Rome, but as she's preparing to leave, Mrs. Baylock appears in the hospital room and throws her through the fourth-floor window.

In an ancient Etruscan cemetery, Robert finds a jackal carcass in Damien's mother's grave. In the plot next to it is a child's skeleton with a shattered skull. Robert realizes that the jackal is Damien's inhuman mother, and that the child in the plot next to her is his own murdered son, killed so Damien could take his place.

A pack of wild Rottweilers attack Robert and photographer Keith Jennings in the cemetery, but they manage to escape.

Bugenhagen, an elderly English archeologist, gives Robert seven sacred daggers to kill Damien with them. He also tells Robert that Damien will have the Mark of the Beast, three sixes, somewhere on his body if he really is the Antichrist. Believing Damien to be an innocent child, Robert throws the knives away. Jennings goes after the daggers, vowing to kill Damien himself if Robert won't, and is killed in a freak accident when a sheet of glass slides off a runaway truck and decapitates him.

Robert discovers the birthmark hidden under Damien's hair, confirming that Damien truly is the Antichrist.

After overpowering Mrs. Baylock, Robert throws Damien into his car and speeds off to the local church. A police officer sees Robert speeding and pursues him. Robert arrives at the church with several police cars in pursuit. He drags Damien inside, kicking and screaming, and takes him to the altar to kill him with the daggers. Just as Robert is about to stab Damien, the police arrive, and an officer shoots Robert.

The film ends with Robert and Kathy's funeral in Arlington National Cemetery. Damien is revealed standing between the First Lady and the President of the United States. Damien turns around and smiles sinisterly at the camera as the prophecy of the Antichrist's rise to power is being fulfilled.

Screenwriter David Seltzer planned to name his antichrist Domlin after the "total obnoxious brat" child of a friend, until his wife convinced him that it would be a horrible thing to do to the kid.

The Omen (1976) was Donner's break-through film, but he would go on to direct a number of blockbusters including Superman (1978), The Goonies (1985), and Lethal Weapon (1987).