Here's a list of all the airlines which fly to Muscat:
http://www.omanairports.com/home.asp?pid=101
I've been trying to work my way through that list to see what's available. The answer appears to be very little! (Please note that I've not checked fares with Oman Air because their online booking facility is not currently available).
Most of the airlines listed don't have any flights available and, generally, those which do are pricey. (e.g. Swiss International Airlines can get you there, via Zurich, for �1303 return).
BA looks to be your best bet. Fly out from Heathrow at 0925 on Friday 23 December for �221, arriving at Muscat 2210.
Now an exact two-week visit would mean returning on Friday 6 January. BA's return flights leave at 0015 (arriving Heathrow 0630) so you'd have to decide whether to leave at 0015 on Friday 6th or 0015 on Saturday 7th. The problem with both of these flights is the cost of �761. If your commitments in Oman permit, I'd suggest coming back a little earlier. If you return at 0015 on Thursday 5 January (i.e. Wednesday night/Thursday morning) the reurn flight will only cost you �245. Ok, it's a day or two less but it's also a saving of over �500!
http://www.britishairways.com
Hoping this helps,
Chris