Forty-eight years following the release of The Omen, the horror film's prequel, The First Omen, will premiere and reveal what happened before the first movie's events. The Omen debuted in 1976 and told the story of Damien Thorn, a young boy who was the prophesied Antichrist. Damien was switched at birth after his (unbeknownst to him and her) adopted mother, Kathy, gave birth to a child who died. The hospital chaplain, Father Spiletto, convinced her husband, Robert, to adopt another baby (Damien) whose mother had passed away during childbirth. Robert did so but failed to tell Kathy about his actions. Unfortunately for the couple, Damien came with a lot of baggage.

https://screenrant.com/first-omen-prequel-set-timeline-when-take-place/