This anti-communist, pro-Western redoubt on the Arabian peninsula's southern shore is convinced of Soviet complicity in the November assault against the Grand Mosque in Mecca, deepening belief here that the Russian bear is on the march in the Persian Gulf.

Oman's leaders claim their intelligence leaves no doubt: the Mecca attack was hatched in communist South Yemen under Soviet auspices as an audacious effort to overthrow the conservative royal government of Saudi Arabia, the world's richest oil state. To Oman, this represents ultimate arrogance in the Kremlin and blantant disrespect for the United States in its Iranian ordeal.

Oman's answer is to intensify its appeal to the rich but nervous Persian Gulf hereditary monarchies and to Western powers particularly the United States, for new aircraft and ships to patrol the strategically vital Straits of Hormuz against the expanding Soviet presence. But beyond hardware, the Omanis want a tougher U.S. posture since the United States permitted the shah of Iran to fall and has suffered the humiliation in Tehran.

While the Mecca assault stunned all Islam, it especially disconcerted the conservative sultanate of Oman, thanks to reports from its intelligence service (which, like its armed services, is run by British). Those reports reject the official Saudi attribution of the attack to religious fanatics not connected with any foreign power.

Omani intelligence contends that the cadre for the Mecca assault was trained in the People's Democratic Republic of Yemen (South Yemen), the Soviet outpost on the Arabian peninsula. Although some arms were of British manufacture for the purpose of "deniability," they were supplied from South Yemen. Furthermore, Omani intelligence contends that the attack on Mecca, if successful, was to followed by uprisings at Medina and "other places in Saudi Arabia."

Similar reports come from intelligence services of other nations, and even some Saudi officials have hinted as much. While U.S. officials reject the Omani report as evidence of a tendency here to find Russians under every bed, they concede the Mecca terrorists got arms from South Yemen.

Whatever the degree of Soviet complicity, it was taken here as evidence of Moscow's arrogance. "It stunned me," one Omani official told us. "I would think the Russians would want to ensure the reelection of a weak president like Carter, or better still, Kennedy and not make any trouble. It shows just how arrogant they are."

Physical evidence of that arrogance is the growing Soviet naval presence here. According to Omani intelligence, eight to 12 Soviet nuclear submarines are berthed in permanent pens in South Yemen alongside several surface warships.

Until recently, a Krivack-class Soviet destroyer (bristling with electonic listening equipment) was on station in the Straits of Hormuz, through which passes Perian Gulf oil destined for the Western world. Lt. Tom Hammon, British commander of one of Oman's two missile-firing fast patrol boats, told us it was recently replaced by a Kotlin-class destroyer. "It doesn't matter," he said. "They've always got somebody out there."

With the Iranian navy departed following the shah's fall, the only counterweight is Oman's competent but tiny navy, which patrols the straits. Accordingly, Sultan Qaboos, Oman's 38-year-old pro-Western ruler, has asked financial help the Gulf states and the West to buy patrol boats, mine sweepers, helicopters and patrol planes.

There was sharp public criticism from leftist, heavily armed Iraq, which decries Oman's breaking of Persian Gulf solidarity to endorse the Camp David accord. The Gulf's jittery hereditary states backed away from Oman after Baghdad's blast. But Omani oficials claim many Arab states -- including Iraq -- privately expressed interest in helping Oman police the straits. "I can assure you," one official here told us, "we want no part of help from Iraq."

It does want help from the United States; an aid package of defensive arms has been approved in the State Department. The recent U.S. delegation seeking emergency basing facilities received a warm reception here.

Oman's position is a welcome for the U.S. naval presence "just over the line of the horizon." Such force -- out of sight, but nearby -- reassured this thinly populated (around 500,000) nation whose armed forces, through numbering only about 15,000, are excellent by Mideast standards. Nevertheless, the sultan's government remains uneasy as it compares the Kremlin arrogance with Jimmy Carter's restraint in this cauldron of world conflict.