Effective November 1st, visitors to the Sultanate of Oman need to apply for an e-visa before heading over in order to "facilitate easy and efficient processing at the border posts," according to a recent tweet by The Royal Omani Police. They went on to state that getting a visa upon arrival is now going to be an exception and that visitors of all nationalities should be applying for e-visas ahead of their travels.
The government has an online portal through which you'll need to apply by scanning your passport and a second photo of yourself. After four days, you'll be granted a 10-day unsponsored visitor visa. The cost of the online visa depends on your nationality, but rill range anywhere between $13 and $130; e-visas are free for visitors coming from Dubai or Qatar.
Border patrol will not allow those who haven't received their e-visa in advance, so don't go and try your luck.
Source: The National UAE