Sultan Qaboos bin Said is an unlikely volunteer for the job of containing communism in Africa and the Middle East.

His name is no household word in the West. Nor is Oman, the thinly populated country he rules.

Qaboos is young (mid-30s) and inexperienced in great power affairs. He leads something of a sporting life as a horseman and yachtsman and commands no arsenals of massive retaliation. His 14,000-man army is lightly equipped and modestly trained. Most of the pilots in his mini-airforce are hired hands from Britain and Jordan.

No matter, Qaboos, in the words of a British adviser, is taking "a very advanced position" against the Soviets and Cuban military adventures in the Horn of Africa. He talks tougher than the president of the United States. His aim is to rally other potentates of the Persian Gulf - the Saudi Arabian king, the shah of Iran and the sheikhs and princes of the United Arab Emigrates - to sound the alarm that the Russians are coming, that they threaten the stability and the oil of the Middle East and the security of its vital sea lanes.

An Americans diplomat here is somewhat bemused by all this. He regards the anti-Communist stance of the Omanis as "an obsession" reflecting an old-fashioned cold war mentality which sophisticates in the United States have outgrown.

"But," he says, "Oman has just been through a Communist-backed insurgency and they view events in the Horn through that filter."

This insurgency was mounted by dissident tribesmen with material and ideological support from Moscow. It has now run its course. But the Russians still sit in South Yemen, the Washington Post's Thomas Lippman reported from African last week, "remains the Soviet Union's most faithful ally in the Arab world." The country has become an armed camp and a major logistics base for the Soviet-Cuban military operation in Ehtiopia.

According to U.S. and Omani intelligence reports, the situation in South Yemen is thsi:

The Russians have poured into the country large quantities of modern weapons, including tanks, heavy artillery, advanced aircraft and armored trucks. They have brought in more than 3,000 Cuban troops and hundreds of East Germans who are training Yemeni soldiers and setting up an internal security system. At the same time they have made South Yemen into a staging and resupply base for the Ethiopian operations, using the magnificent port of Aden which is less than 200 miles from the African continent.

The Omanis claim that at least 3,000 South Yemeni troops have been sent off to Ethiopia to join the fight against Somalia.

American officials in Oman and Washington are watching all this with detachment and apparent equanimity. A Pentagon official said last week that nothing the Russians are doing in the Horn of Africa or in South Yemen represents "an immediate strategic threat to the United States."

But to the Omanis and other states of the Gulf the threat is neither abstract nor academic. There is apprehension in government circles in the area that that United States despite its dependence on Arab oil may not be a reliable or credible military protector.

That is one reason Oman in 1976 devoted 70 percent of its budget to "defense and national security." And that is one reason Saudi Arabia is trying to buy modern weapons including F15s from the United States.

For hundreds of years Oman lived in isolation from the West. It was a maritime power dominating the trade routes to East Africa, India and the Persian Gulf. In the 16th century the European invaders arrived, first the Portuguese, then the British and now the Russians and Americans with their global interests.

Oman today is an anachronism, a weak nation with 775,000 people struggling to enter the 20th century under the rule of one of the world's last sultan. It is caught up in an international power struggle it cannot control or significantly alter.

Its government is nervous. There is Marxism on the Western border. Alien armies are marching in Africa. There are rumors of war and revolution throughout the Middle East. Qaboos sounds the alarm. The Americans are unmoved.

So Oman looks increasingly to other Gulf states for protection, notably the Iranians who may in the end intervene in the Somalia-Ethiopian conflict.