Becoming disillusioned by his British Army service, in particular his career prospects, he spent the last two years of his service seconded to the army of the Sultan of Oman. At the time, Oman was experiencing a growing communist insurgency supported from neighbouring South Yemen. Fiennes had a crisis of conscience soon after arriving in Oman, as he became aware of the Sultan's poor government. However he decided that the oppression threatened by a communist takeover, combined with moves towards progressive change within the Sultanate system, justified his part in the conflict. After familiarisation, he commanded the Reconnaissance Platoon of the Muscat Regiment, seeing extensive active service in the Dhofar Rebellion. He led several raids deep into rebel-held territory on the Djebel Dhofar and was decorated for bravery by the Sultanate. After eight years' service Fiennes relinquished his commission on 27 July 1971.