The sultan of oil-rich Oman arrived in town yesterday bearing a $300,000 gift that should be music to the ears of National Symphony Orchestra supporters. In a gesture unparalleled by visiting kings, presidents and potentates, Sultan Qaboos Said Said endowed a chair of narrative music with the NSO in First Lady Nancy Reagan's name.

Long a fan of Mozart, Brahms, Haydn and Bach, the sultan apparently added Mrs. Reagan to his list of favorite artists shortly after she performed at the orchestra's March 28 pension fund concert. She narrated Ogden Nash's verses to Camille Saint-Sae ns' "Carnival of the Animals."

When Mrs. Reagan heard the news, according to Sheila Tate, her press secretary, she could not have been more pleased. "I'm very touched that the sultan chose to honor me in such a thoughtful fashion and one which will help young performers," Mrs. Reagan said in a statement released by Tate.

"This extraordinary grant to the National Symphony Orchestra, the first directly made by a head of state," said Leonard L. Silverstein, NSO president, "reflects the exceptional generosity of the sultan and the people of Oman, as well as the generous support which Nancy Reagan has made to private initiative in support of the arts. Not only does the grant symbolize the world-class status of the National Symphony Orchestra, but it also plays a significant role in enabling us to meet our current financial needs."

Last night at the Madison Hotel, where he's staying, the sultan met privately with former president and first lady Jimmy and Rosalynn Carter. Tonight he'll dine with the Reagans at the White House and be entertained by opera stars Robert Merrill and Anna Moffo. Among those in the East Room audience will be another singer -- Ethel Merman. CAPTION: Picture 1, Qaboos Said Said, who gave the NSO a $300,000 gift for narrative music; Picture 2, Jimmy Carter and the sultan of Oman yesterday; by Harry Naltchayan