If you have a car hired from the UAE you need to have insurance from the UAE and (usually for a hire car) also a separate permission letter.
I can confirm the need for a formal letter from the rental company authorizing you to take it out of the country. Also, you will need to pay for an Omani visa at the border as well (presuming you're from a western), so have some cash on you. :) I think currently it's about $52.
Thanks for the info folks!
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