The government has issued its highest alert level of red for Torba, Sanma, Penama and Melampa provinces.
Oma is still a category two system and was sitting 145km west of Santo at 8am local time this morning.
It is heading in a north north-east direction towards Santo's north-west, with winds as high as 130km/h at its centre.
Authorities in Vanuatu are warning of torrential rain, damaging winds and violent storm surges.
The Meteorological Service has described the sea swells in the area as phenomenal and is urging people to stay inland.