Gregory Peck did not receive an Oscar nomination for portraying Robert Thorn in The Omen.
In the seventies many of the old time Hollywood actors find themselves in parts and films that were quite the far cry from their earlier films. This was certainly the case for Gregory Peck being in this film. Being in a film of this nature could easily lead such an actor to go BIG, which Peck would do two years later in The Boys From Brazil, although I must admit that I, and apparently everyone else, like that performance. That's not the case here for The Omen as Peck does not use it as an excuse to play into the rather extreme and in someways absurd tone of the story. Peck instead really downplays his part as the American diplomat in England who after the death of his son, apparently in childbirth, he is offered oddly a new one by a questionable priest. Peck's actually really quite good in providing just the honest emotions of the situations as he conveys Thorn's confusion over the request while realizing the certain emotional vulnerability in him at the time which makes him conducive to the strange request of the priest to take this other child as his own.
After this point he and his wife (Lee Remick) seem content enough with "their" child Damien this until some strange this start to occur starting at Damien's birthday party where his current Nanny hangs herself in front of the party claiming it was all for Damien. This scene sets up the point of much of Peck's performance in the film as Thorn is one of the many witnesses to the hanging. That being quite a very down to earth human reaction to the very bizarre occurrences that all seem to surround his adopted son in some way. Peck's very good in this scene in making the horror feel real through his own realistic reaction fitting a man who has just scene a woman hang herself apparently as a tribute for his son. This actually goes for every death in the film as Peck gives them a bit of extra weight past merely the shock factor by giving some humanity to them in addition to the shock factor. The majority of this comes from Peck who avoids falling into overacting and going in with some of the ridiculousness of the kills, but rather makes them far more horrifying by showing what a normal man's reaction would be to them.
Peck's performance though does go past simply giving something to the film's more extreme moments of violence. He also interestingly acts as one of the two straight men for the film, the other one being David Warner as a photographer whose photographs tell him more than he would like. The arc for Thorn begins when that same priest who gave him the baby now appears to him telling him that the child is in fact the antichrist and must be destroyed. Peck does well to begin as one would expect which is simply sheer disbelief and confusion over these revelations, as they seem to be complete nonsense. When the odd yet tragic events begins though things begin to change. Peck is very effective in portraying the gradual change in his character. As that initial confusion becomes more of a concerned puzzlement as problems continue to occur, to slowly something more as it is evident that it clearly has something to do with his "son". Peck manages to find just the right natural approach in this as it never feels as though Thorn is being unrealistically stubborn, or far too easily accepting of such otherworldly ideas.
What Peck does particularly well is keeping the revelations with the severe attachment that this involves the boy he has raised himself, and Peck carefully keeps this alive in a certain pressure in Thorn explaining what keeps him from accepting the truth the way Warner's character does. Certain things work their way to get Thorn to move back this though, when the violence begins hitting closer to home. Peck is really quite moving in just the quiet despair he reflects when Thorn is told some especially horrible news over the phone. Peck is able to attach this within Thorn to almost wholly accepting the truth of it all, but unfortunately there is one problem to solve it all he must kill the boy. I'll admit a slight sour point for me comes from soon after the news as I feel Peck overacts just a tad in portraying Thorn's rejection as just a bit much, to me it felt like Peck in Spellbound which is not a good thing. Thankfully Peck more than makes up for it in the climatic scene where he realizes the terrible conflict in Thorn that forms his hesitations but also his convictions as he attempts to commit the deed despite it seeming so unbelievable. This is a strong performance from Peck as he manages to ground the film by keeping a human element within it, while stopping it from becoming simply a fright show.