The Omen was created on 1976-06-06.
Merkaz Omen was created in 1958.
Secret Omen was created in 1978.
Earth Omen was created in 1972.
The Omen Machine was created on 2011-08-16.
Omen - song - was created on 2009-02-16.
Omen - Star Wars novel - was created in 2009.
Omen III - song - was created on 1994-01-14.
The Pious Bird of Good Omen was created in 1967-09.
There have been at least five Omen films. The Omen(1976), Damien: Omen II (1978), Omen III: The Final Conflict (1981), Omen IV: The Awakening (1991) (TV) and The Omen (2006), which is a remake of the first movie.
Omen
It depends on context. "The meteors were an omen of pending disaster" or "the patrol's appearance was an omen of more to come" are both acceptable.
Ominous: [Latin, ominosus < Omen] Of or being an omen, esp. an evil one. So, yes, ominous is the adjective form of omen.