A red flag, the color of the Kharidjite sect of Islam, was common to the states of the Arabian shores of the Persian gulf and their colonies in the Indian Ocean from the early 19th century to modern times, but they have all since modified it in one way or another.
It was plain red in Oman until 1970 when the present Sultan introduced a complete new set of national flags, that were to be hoisted on national flagpoles.
These flags make use of the traditional emblem of the Sa'idi dynasty, which is a belt with a dagger superimposed on two crossed swords. This was placed in the canton, and panels of white and green were added to the fly.
The white part is supposed to represent the Imam of Oman, who raised a white flag in 1954 when he tried to set up a separate state in the interior. The design was slightly modified some three decades later.
This flag is available for purchase here.