When day came at last the hobbits were surprised to see how much closer the ominous mountains had already drawn.
The Riders had returned to their silent ominous vigil.
Then Frodo and Sam staring at the sky, breathing deeply of the fresher air, saw it come: a small cloud flying from the accursed hills; a black shadow loosed from Mordor; a vast shape winged and ominous.
The mountains still loomed up ominously on their left, but near at hand they could see the southward road, now bearing away from the black roots of the hills and slanting westwards.
There, far away, beyond sad Gondor now overwhelmed in shade, the Sun was sinking, finding at last the hem of the great slow-rolling pall of cloud, and falling in an ominous fire towards the yet unsullied Sea.
Then suddenly the cry of brazen-throated trumpets was heard: from the watch-towers they blared, and far away from hidden holds and outposts in the hills came answering calls; and further still, remote but deep and ominous, there echoed in the hollow land beyond the mighty horns and drums of Barad-dur.
There are no more uses of "ominous" in the book.
Show samples from other sources
I’m beginning to hear ominous rumblings of discontent.
With the clouds looking so ominous, I’m afraid there will be a bad storm.