The carrier earlier this year revealed it had signed a letter of intent to take up to five A330s to support its resumption of long-haul services.
It has now signed a firm contract for five A330s with Airbus. No engine choice has yet been made.
Oman Air Ziad Al-Haremi says: "Our decision to buy the Airbus A330 all came down to the suitability of the aircraft with our long term strategy to operate an efficient and reliable fleet with proven operating costs."
The carrier has previously said it would take delivery of the aircraft in 2009, though it has leased in Airbus A330 and Boeing 767 capacity to enable it resume long-haul flights sooner. These are due to begin at the end of the month with new flights from Muscat to London Gatwick and Bangkok.