In 1965, "two rival nationalist groups--the Front for the Liberation of Occupied South Yemen (FLOSY) and the NLF--launched an uprising against British colonial rule, compelling British rule to end. In 1967, in the face of mounting violence, British troops began withdrawing and the federation they set up collapsed. Later that year the NLF eliminated its FLOSY rivals and declared South Arabia, including Aden, independent on November 30, 1967, subsequently naming the new state the People's Republic of South Yemen."
By the way, most older Omanis today [in 2015] still refer to the south and eastern half of of Yemen as "Adan", which is basically where the Peoples of Republic of South Yemen was located till 1990.
Outside of Oman, a third group of living-in-exile-citizens of Oman were growing up in both Eastern and Western Europe during the Cold War, but both groups--whether trained and reared in the pro-capitalist West or in the communist East--looke forward to the day when they could return to their homeland and finally build a modern Oman. Naturally, the lives of these Omanis were vastly different sets of experiences than those of their own people, family, and tribes back in their homeland.
One major example of the internationally educated Omanis seeking to return home in some distant dreamlike Omani future was Sultan Taimur's own younger brother, Tariq. Alas, in 1966 when the last major assassination of Sultan Taimur occurred, Tariq bin Taimur, self-exiled for several years, only launched belatedly "a movement aimed at overthrowing Sultan Said and restoring democracy. Born of a Turkish mother, married to a German, speaking five languages fluently, Tariq had friends in the Western embassies in Kuwait and Beirut, and was thinking of raising a mercenary force to remove Said from power. So trouble was brewing." meanwhile a full-scale war was under way in the Dhofar region of Oman, where Salalah was still the capital of the Sultanate.
Kutschera has called the Dhofar Insurgency or the last Omani civil war of 1962-1976 , the country of Oman's "Dirty War". In Dhofar, the Marxist rebels of the Liberation Front of Dhofar virtually controlled the hills, mountains and even the small coastal villages. The Sultan controlled nothing but Salalah, which had become an entrenched camp. Until 1972 no reporter was permitted to visit the region or the conflict.
In the early 1960s the Iraqis began to back and train the Sultan's enemies leading to the supporting of the communists who eventually took over all of the mountains of Dhofar, which surrounded the capital of Muscat and Oman. American marine analyst, Steven A Cheney, has written the following concerning this period of Omani history in his The Insurgency in Oman 1962-1975:
"In 1962 a semblance of organization emerged among the insurgents. The Dhofar Charitable Association (DCA) was established, ostensively as a cover for the Dhofar Libera- tion Front (DLF). The DLF was associated with the Arab Nationalist Movement (ANM), a Nasserist organization that had branches in virtually every Arab state. The ANM was proving to be a particularly effective organization, having sponsored successful revolutions in the Yemens. However, their expressed goal was to defeat British imperialism3 without any ties to an Imam. Two other groups, the Dhofar Benevolent Society (DBS) and the Dhofari Soldiers Organization (DSO), were beginning to form. Both operated underground in the vicinity of Salalah until early 1965, when a shipment of men and arms from Iraq and Kuwait was intercepted by the Iranians. As a conse- quence, many of the insurgents in Oman were captured, and those remaining fled to the mountains. They met at Wadi at Kabir on June 1, 1965, to solidify the leadership of the DLF and prepare plans for an extended campaign. That meeting was declared the "First Congress" and produced a proclamation demanding the liberating of this country (Dhofar) from the rule of the despotic Al Bu Said Sultan whose dynasty has been identitied with the hordes of tne British imperialist occupation.... This people (Dhofaris) have long and bitterly suffered from dispersion, unemployment, poverty, illiteracy and disease.... The DLF also issued a three point manifesto stating, a. The poor classes, the farmers, workers, soldiers and revolutionary intellectuals will form the backbone of the organization. b. The imperialist presence will be destroyed in all its forms--military, economic and political. c. The (hireling) regime under its ruler, Said bin Taimur, will be destroyed.
After the Arab-Israel war ended in late 1967, Cheney notes, the Dhofar insurgency became much more international and a communist takeover of the country loomed larger--as the tribal leaders were marginalized within the ranks of the insurgent military ranks.It is at this juncture that the insurgency began to take on an international flavor. The indigenous tribesmen lost out as a controlling faction. Communist leanings trended to stronger ties with the new revolutionary regime in South Yemen, and garnered support from the Peoples Republic of China, Iraq, and radical Palestinian organizations. After liaison was made with the Soviet Union, several young leaders attended school there.10 The situation was not dissimilar to that faced by South Vietnam. A friendly communist government immediately bordering the country provided profuse supplies and safe havens for guerrillas."
Considering that Maoists in China of the mid-1960s were on a turn-back-the-clock binge, i.e. which has been misnamed a "cultural revolution", it is doubtful whether Oman could ever have become a well-developing nation if those types of communists had been successful in taking over Oman. In short, in many ways, the Cultural Revolution in China from 1966 through the 1970s was extremely anti-intellectual and downplayed technical achievements and performances--this was exactly the opposite of what Omanis would want to see from governmental leadership in the coming decades. In short, even though either (1) the Maoists or (2) the Saudi backed (and religiously oriented) forces in Dhofar and other Omani regions might have offered a better future for the peoples of Oman in contrast to the ongoing Taimur regime. However, the religious extremists and the Maoists in the end were not likely to educate their citizens better nor as well to provide as much health care as the Omanis have received over the past 4 decades under the current Sultan. 
WHY DID SULTAN QABOOS SUCCEED SO WELL?
A few weeks prior to the Omani military initiated Sultan Qaboos Coup of the 23rd of July, 1970s, "most observers in the Persian Gulf and Beirut thought the fall of Sultan Said [of Muscat and Oman] was only a matter of time: 'When the oil revenues reach 50 million ($120 million) the British will remove him and put a Tariq in power', said one well-placed source in Abu Dhabi last June. Said bin Taimur was the sick man of Arabia; intervention was imminent, and Tariq [the brother of the Sultan] was thought of as the surgeon."
"Why did the British choose to let Qabus' coup succeed [instead]?" asks Kutschera "Was it, as a British diplomat in Muscat had the audacity to say, because 'it (British policy) is not to interfere with the local affairs of the natives'? Or, as other say, was it because Tariq was too involved abroad or because of his idea of democracy, which would give the Sultan a largely honorary role? Or did the new Sultan really impose himself?"
In retrospect, the coup of 1970s simply had perfect timing. It came at a juncture when Taimur would have likely had to flee anyway. He simply had no one left in the kingdom who supported him.
However, just as important as timing seems to have been the ability of the young Sultan Qaboos to convey the message to his own people and to the British forces who eventually backed him (during the last years of the insurgency in Dhofar) that he was in charge.
Likewise, J. E. Petersen writes that at the same time as the coup occurred, most Dhofaris in the mountains were becoming angrier and angrier with the hardline Soviet and Maoist leadership. "Most Dhufaris were won over by a combination of the obvious commitment of the new regime [under Sultan Qaboos] to development and a better life, and mistakes by the front's hardline leadership in its sometimes brutal suppression of Islam and tribalism.