Armand Hammer, art collector, oilman, gift-giver and billionaire, yesterday presented 1,000 Daumier lithographs worth $150,000 to the Corcoran Gallery of Art, and then--to the gallery's astonishment--added a surprise:

"In addition to the thousand lithographs," said Hammer, "I am also donating $100,000 so the museum can continue to add to its Daumier collection, and also buy works by contemporaries of the artist."

"I must tell you," said David Lloyd Kreeger, the gallery's board chairman, "we had no advance notice of the added $100,000 gift--which you just heard announced in a throwaway line. In all my years I have never encountered a patron with such spontaneous generosity."

Hammer, 84, a gallery trustee, announced both of his gifts during a press conference in the Corcoran's auditorium, a hall he paid to restore and that now bears his name.

An earlier Hammer present--of nearly $1 million in 1979--allowed the museum to dispense with its admission fees.

"Five Centuries of Masterpieces," a show of paintings drawn from Hammer's personal collection, was exhibited at the Corcoran in 1980. His Leonardo notebook, now called the Codex Hammer, received its first public showing there in 1981. (The codex, still on tour, will be in Baltimore at the Walters Art Gallery from May 14 through Aug. 28.) In 1979 the Corcoran displayed a batch of his Daumiers. The next year he gave the Corcoran 18 Daumier bronzes, worth $250,000.

He has been as generous to other art museums. His finest master drawings have been pledged to the collection of the National Gallery of Art. He has promised his Leonardo codex, many of his paintings and most of his Daumiers to the Los Angeles County Museum of Art.

Hammer avidly collects the art of Honore' Daumier (1808-1879), the French printmaker and painter and lampooner of the powerful. Since he started buying Daumiers 13 years ago, Hammer has assembled what is probably the largest privately owned collection of the artist's work. He owns more than 6,000 Daumier lithographs, nine oils, 20 drawings and watercolors and 38 small bronzes.

In purchasing voraciously, in buying whole collections, Hammer has acquired multiple examples of many Daumier prints. It is these duplicates, these seconds, that are coming to the Corcoran, for the bulk of his collection is going to Los Angeles, to a soon-to-be-established Daumier Study Center.

The 1,000 Daumiers would knock them out in Phoenix or Toledo. Why the Corcoran should want them is not exactly clear.

It is, after all, a gallery "dedicated to the American genius." Though many years ago it acquired a number of fine European pictures (among them two Daumiers, an oil and a drawing), it has never spent its monies on 19th-century French art. In 1979, in fact, the Corcoran auctioned off scores of European paintings, many of them French, in order to raise purchase funds for American works of art.

And Daumier is already thoroughly represented in other Washington museums.

The 18 Daumier bronzes Hammer has given to the Corcoran come from a set of 36, all relatively common. They are 20th-century casts of Daumier's small clay caricatures of French politicians, and not one is unique. The National Gallery, in fact, owns a complete set. The Hirshhorn Museum and Sculpture owns casts of all but three.

Daumier's lithographs are also common here. The National Gallery owns 920, the Phillips Collection has 49 and the Library of Congress possesses 33.

On the other hand, Daumier is remembered as the father of political cartooning. In accordance with the principle that "if a little bit is good, then a lot is a lot better," Daumier's bitter prints probably deserve display in a capital like ours. Many of the 1,000 prints that Hammer has given to the Corcoran zap generals, lawyers and pompous politicians. Though made between 1840 and 1870, they have not lost their bite.

Hammer's $100,000 gift will come to the Corcoran in four annual installments. Many of his Daumier lithographs will be shown in an exhibit there next fall.

Hammer, who was trained as a physician, began accumulating his fortune while visiting the Soviet Union in 1921. In 1957, he became president of the Occidental Petroleum Corp., then a minor oil-drilling firm. Occidental, for which Hammer now serves as board chairman and chief executive officer, recently merged with the Cities Service Co. It is now the 15th largest American corporation.