Airlinefleets.net data provided for the Oman Fleet chart below
Since the purchases of the A-330's from 10 years ago, Boeing has done a lot of cooking in its industrial kitchen. Even though Oman's A-330-200 has received upgrades in its fleet a taste for the 787 spoils its fleet composition. Having a propensity for 737 NG's at 35 units, the transition for more 787's is a natural expansion for its wide body division.
An A350-900 for Oman maybe more aircraft than Oman needs and the 787-8 could be obtained for a low price when buying 9 of its type replacing any aging A-330's and seeking a fleet commonality for its single aisle division.
Oman has stated its seeking the best price in the competition and will know in three or four months what the decision will become. It is a just in time announcement for the Dubai Airshow where it can make the most publicity out of a wide body purchase.
Logic suggests a Boeing deal in the offing. Logic also says an Airbus deal is possible at the right price and Airbus is starving for sales this year for its A-350's booked. The hint is out that Boeing may have big announcements coming forward and Airbus would like to steal one from Boeing in the worst way. Boeing also is on a a "down year roll" and would like to keep the numbers coming in as it could throw sweeteners Oman's way since it has a solid 737 NG fleet. If the 787 in Oman's fleet has proven itself worthy during its operations, and Boeing makes a great deal possible for Oman's fleet line-up from single aisle to dual aisle then the wide body announcement will fall Boeing's way and Airbus can do little to stop Boeing sales momentum it has achieved during 2017.
Even though, "counting chickens before they hatch", is a silly thing to do Boeing has the leveraged position for this opportunity. Airbus will have to give away some wide body orders to win the day.