Until 1975, Oman used the plain red banner of the indigenous people. In 1970, the Sultan introduced a complete new set of national flags. Bands of green and white were added to the fly, and the national emblem, the badge of the Albusaidi Dynasty, was placed in the canton. This depicts crossed swords over a khanjar, a traditional curved dagger. The countries mentioned in the other answers also had red flags, but decorated as follows:

-- The flag of the USSR also had on it a hammer and sickle.

-- The flag of the PRC also has five gold stars.

-- The flag of Albania also has a silhouetted black double-headed eagle.

More Info: en.wikipedia.org