I was there a few weeks ago (cw out , wtp back). All of above is good advice and captures things well. There are no airbridges yet so will be a bus, and the lounge is adequate but nothing to write home about.
I would say (if it works - I wish I'd known about it ) the E-visa is worth looking at. The airport is a bit of a (friendly and nice) scrum and it's a short overnight, so anything to save hassle is worth it. They will also sting you at the desk on the foreign exchange if need to pay by card etc.
Enjoy your first first and also muscat , it's a wonderful place!