The answer
The airport's own transport page lists Mwasalat public buses from the bus station on the ground floor outside the terminal — route A1 threads Al Mabelah–Airport–Ruwi bus station, roughly 45 minutes to the capital's hub for a fare around OMR 1.000.
Muscat has no metro or train yet (the planned 55 km Sultan Haitham City–Ruwi line via the airport is still on the drawing board), so the A1 bus is the only fare-cheap way in and it runs day and night.
Tight budget? A1 to Ruwi at about OMR 1.000 (Mwasalat has been running a 50%-off promo on A1 airport service) — then short hops on city lines at 300–500 baisa.
With luggage or going to Muttrah/Qurum? Licensed orange-white taxis queue outside arrivals; the meter is mandatory and a city-centre ride lands near OMR 9–10.
⚠️ The traps (walk past these)
The forecourt is not a waiting room — OMR 2.100 per 10 min after the free 10
Published airport tariffs: pick-up/drop-off vehicles wait free for 10 minutes outside the departure terminal, then it's OMR 2.100 for every further 10 minutes (short-term car park: OMR 0.600 up to 30 min, OMR 1.100 to 1 hr). A friend circling while you clear immigration racks up a fine-sized bill.
Do this: tell pick-ups to park in the short-term lot (OMR 0.600 first half-hour) and only edge the forecourt when you're through customs
'Old Terminal' on timetables — that's the low-cost shed, not where your flight lands
Everything moved to the new 580,000 m² terminal on 20 March 2019; the revamped old building handles low-cost carriers. Moovit and older guides still show an 'Old Terminal' bus stop — if your airline is a mainline carrier, the A1 airport stop is at the new terminal, and vice versa.
Do this: match your terminal (new vs LCC/old) against your boarding pass before walking to the bus stop
Cash on the bus, rials on the meter — no card, no exceptions culture
A1 tickets are bought on the bus in Omani Rials; guides report small notes or exact fare, and the Mwasalat app's ticket sales have a mixed record. Many taxi drivers will take a card but the airport's own parking machines state OMR cash only — the same cash-first logic applies at the kerb.
Do this: withdraw or exchange a few OMR notes at the arrivals ATMs before the bus stop
No rail link exists yet — 'Muscat Metro' news is a plan, not a ride
Headlines about a 55 km metro through the airport (Sultan Haitham City–Ruwi, ~42 stations, French MoU signed) are real — but it is future infrastructure. A guide or blog telling you to 'take the metro to Ruwi' is describing a construction project, not today's transport.
Do this: budget road time: bus or taxi, 30–50 minutes, no train option