Trans-Gulf, you can get accomodation near the airport and decent accomodation at that for around OMR 600. the area you mentioned was Al-Hail and you also have Mawahla and Seeb. The best bet it seems at the moment is to actually drive around the areas and ring the numbers that the landlords have outside the vacant buildings. A lot of the landlords seem to shun the letting agencies, and rent themselves.
As for the yearly rental, Oman Air only pay 6 months in advance, however they do take out a yearly contract. And judging by the way its going at the moment the landlords seem happy with that, simply because they know that the rental is coming from a company and not an individual.
I dare say if you were to take the housing allowance off Oman Air and rent yourself, you would probably have to fork out a year's advance!