would this be PG-13

Depends on which country you live in. Some rating systems are stricter than others:

Argentina:18 (original rating) / Argentina:13 (re-rating) / Australia:MA (25th anniversary special edition) / Australia:R (original rating) / Brazil:16 / Canada:R / Canada:13+ (Quebec) / Canada:R (video rating) / Finland:K-18 (uncut) (2001) / Finland:K-16 (cut) (1992) / Finland:K-18 (cut) (1976) / France:-12 / Germany:16 / Iceland:16 / Italy:VM18 / New Zealand:R16 (1992 re-rating) / New Zealand:R18 (original rating) / New Zealand:(Banned) (video release, 1989-1991) / Norway:18 / Portugal:M/16 / Singapore:NC-16 / South Korea:15 / Spain:18 / Sweden:15 (cut) / UK:18 / UK:X (original rating) / UK:15 (tv rating) / UK:15 (video rating: special features, additional material) (2008) / UK:15 (video rating: 30th anniversary edition) (2006) / UK:15 (video re-rating) (2006) / UK:18 (video rating) (1987) (1999) / USA:R (MPAA rating: certificate #24560) / West Germany:16

Well the 2006 (many say over - faithful) remake was (still) a USA R.🐭

Not with that decapitation scene. Hell, even without it, the film still might garner an R rating; the MPAA would deem it too dark, thematically speaking.

It used to be rated '18' in the UK, but was re-rated '15' back in 2006. http://www.bbfc.co.uk/releases/omen-1976

YOU SHOULDN'T BE EATING SAUSAGES!!

No way, too much gore.