20 Things You Didn't Know About The Omen

With Holden turning down the project - most likely to shoot Network, which came out around the same time - the studio set their sights on another man-of-action actor who had recently had a brush with a similarly cultish genre. Charlton Heston spent much of the late 60s and 70s appearing in sci-fi films, thanks in large part to the success of The Planet of the Apes. A switch to horror wouldn't be that much of a stretch.

Heston did not give a reason for his rejection, it's possible the role didn't have enough firearms, but given his long connection to Biblical adaptations (from The Ten Commandments to an audiobook reading), he may have found the material objectionable.