The United States is planning an elaborate air, land and sea military exercise in and around the strategic Persian Gulf country of Oman this fall to reassure oil-producing gulf countries that American forces could hurry to their assistance in an emergency, administration officials said yesterday.

The exercise also would serve notice on Iran that it would risk a sharp U.S. response if it threatened moderate Arab nations friendly to America with military force or subversion, diplomatic sources said.

A Pentagon official said the Reagan administration, mapping its policy for that area after the Palestine Liberation Organization withdrawal from Lebanon, is especially concerned about reassuring Saudi Arabia. It is confronted with a militant Israel near its northwest border and an Iranian army attacking Iraq on its eastern flank.

The sultan of Oman, British-educated Qaboos bin Said, has insisted on a low-profile American military presence in his country in the past, diplomatic sources said, but he now seems willing to allow practice landings by Marines on his shore.

If all goes well with the delicate negotiations under way, officials said, the high-profile military exercise would take place in October.

Oman has particular strategic importance to the United States and other oil-importing nations because it is located on the Straits of Hormuz, the Persian Gulf gateway through which oil tankers must pass to reach the open waters of the Arabian Sea.

Like Saudi Arabia, Oman is worried about what Iran will do next. Although its army could shift from Iraq to make direct assaults against Oman, Pentagon officials consider it more likely that Iran's Ayatollah Ruhollah Khomeini would threaten the sultan's rule by exporting Islamic radicalism to Oman, hoping for a coup.

As a result of the Reagan administration's review of its Persian Gulf policy, the Pentagon has put top priority on finding ways to protect such friendly Arab governments from being toppled by radicals. Direct military assaults against Persian Gulf fields are regarded only as a secondary threat in the Pentagon's revised war plans.

Oman also has to worry about Soviet-backed incursions from South Yemen to the southwest. The border between those countries is still in such dispute that soldiers from both countries keep their guns trained on each other from outposts in the rocky hills.

British military advisers are stationed at those outposts to train the Omanis in infantry tactics. The British also have trained the Omani navy and air force.

There are indications that the United States will broaden this western influence by helping the sultan modernize his army, navy and air force. Defense Secretary Caspar W. Weinberger, after visiting with the sultan in Muscat earlier this year, said the two leaders agreed they have a mutual interest in strengthening the Omani military, with the sale of American weapons implied but not specified.

The U.S. military exercise in the planning stage could turn out to be the closest cooperation yet between Omani and American military forces. This has been the pattern of recent military exercises in the Arab world, with the Bright Star exercise in Egypt cited yesterday by administration officials as an example.

During Bright Star, B52s flew from North Dakota nonstop to drop bombs on an Egyptian target range, and then returned to the United States. American troops also conducted joint exercises on the ground in Egypt with Egyptian forces. Joint U.S.-Omani maneuvers during the exercise being planned are a possibility, but Pentagon officials said they could not confirm that at this stage of the negotiations.

Two years ago, Oman allowed American troop-transport planes to use its air base at Masirah during the ill-fated mission to rescue U.S. hostages in Iran. U.S. Navy planes also fly regularly out of Oman carrying mail and key supplies to American warships in the Arabian Sea.