This will apply to all vehicles, not just Omegas
Easiest place to drill holes is on the roof above either the front or rear courtesy lights, making sure that the bolts and cables from the aerial do not protrude too far into the cabin to affect putting the light back afterwards - this will prevent from having to remove the roof lining.
Wing mounted aerials are generally avoided if at all possible, as can be a little directional in their performance, but OK as a last resort and certainly better than any form of glass mounted or internal aerial.
General rule of thumb, keep the aerial, upright and towards the centre of any large metal panels, and before somebody says that internal ones are ok remember that a TV aerial inside the loft will seldom work, yet put outside of the same house in the same location will give a much stronger reception - same is true in cars.
