Oman Air is considering placing an order for up to 13 Airbus A 350-900s as part of a deal to replace its Airbus A330-200/300 fleet from 2020 onwards. Currently, WY operates 6 A333s + 4 A332s + 4 B788s + 6 B789s.
One does not see no justification for this as WY already will be having nearly 10 B787s (788s and 789s) by the time the A359s would get delivered. They dont have such a large fleet to warrant diversification. It is best to stick to one airplane family to meet their needs because if they required the larger capacity A359 to service any route, then they can instead easily order the B787-10 (781).
In this way, their 2020-30 wide body fleet is revolving totally around the B787 family rather than A359 + B787. A standardized fleet in turn will result in synergies that translate into cost savings and better efficiency operationally.