OK, here's a post I think I can answer with a little authority.
There are three major international schools in Muscat (by international, I mean the students are from a number of different nations, some schools describe themselves as being international, when what they really mean is that the staff are from different countires) These are:
BIST (The British International School of Muscat.)
This has a curriculum that follows the NC, the school is aimed at those families that may be coming to Oman for a couple of years, but intend to return to the UK.
ABA (American British Academy)
This follows the International Bac. and is best suited to those ex-pat families that may be travelling from one country to another and need some consistency with the education their children recieve.
TASIM (The American School in Muscat)
This is, as the name suggests, an American style school, that follows an American style curriculum.
If you are only intending to stay for a year or two, and then return to the UK, then maybe the BIST would be the obvious first choice. But if they are full, ABA is a very good alternative (I have been on the PTA).