The GCC countries are all reportedly backing the Saudi intervention in Yemen with the sole exception of Oman. Oman has long had the best relations with Iran of any GCC country, and was the intermediary that originally brokered the nuclear talks; it has also been aloof from GCC plans for a joint military force, so its absence from the Saudi coalition is not a surprise.
Besides, Oman is preoccupied. On Monday Sultan Qaboos returned to his country after eight months in Germany for medical treatment of an undisclosed ailment. For the first time in his 45 years on the throne, he missed the country's National Day (his birthday) in November and addressed the country from Germany.
Qaboos is now the longest-serving Arab leader; he has no children or siblings, and no designated heir apparent; and at 74 he appears frail and much thinner in recent photos (below). Official statements have claimed his medical treatment was a success, but the nature of his illness and the reasons for the eight-month absence remain unclear.