Oscar L. Chapmen, 81, Washington Interior, died of pneumonia yesterday at his Washington home.

He had suffered from a circulatory ailment for some time but had continued to practice law activately in the firm of Chapman Duff and Paul until last summer.

Mr. Chapman became Secretary of the Interior on Dec. 1. 1949, remaining in that cabinet post until the end of the second Truman administration in 1953.

When he left the cabinet, he also ended 20 years of service with the Interior Department. He had been named assistant secretary in the early part of 1932 and at the time was the youngest member of the so-called "Little Cabinet" in the new Franklin D. Roosevelt administration.

Mr. Chapman had gotten that job as a liberal and a New Dealer who had campaign successfully for two Democratic senators from Colorado in 1930 and 1932.

In February 1946 he briefly served as acting secretary of the Interior after Harold L. Ickes resigned. When Julius Krug was named secretary later, Mr. Chapmen became under secretary, a position he held until stepping into the top post when Krug resigned.

He reportedly had accepted the appointment in 1933 because he was interested in advancing the rights of minority groups. In addition to being in charge of Indian reservations, in Interior Department at that time governed Hawaii, Alaska, Puerto Rico, and other areas.

Mr. Chapman worked to improve conditions for the Indians and he endorsed statehood for Hawaii. In 1939, when the Daughters of the American Revolution refused to let Marian Anderson sing at Constitution Hall, mr. Chapman, with the help of Ickes, arranged for Miss Anderson to perform at the Lincoln Memorial.

Mr. Chapman had attended all of the Democratic National Conventions for years. He had been active in a number of presidential campaigns.

Although he was born in Omega, Va., and was educated at Randolph-Macon Academy in Bedford, Mr. Chapman considered Colorado his home.

He enlisted in the Navy in 1918 and served overseas during World War I. He developed tuberculosis and was swnt to Denver, Colo., to recover. He remained there after his medical discharge in 1920.

Mr. Chapman attended the University of Denver and the University of New Mexico. During those years, he also worked as assistant chief probation officer and then chief probation officer of the Juvenile Court in Denver.

He earned a degree from Westminster Law School, now part of the Universtity of Denver, in 1929, when he was admitted to the Colorado Bar. He later received numerous honorary degrees.

Mr. Chapman at one time was on the board of directors of the Pennysylavania Coal and Coke Corp. and had been a registered lobbyist representing the National Union of Sugar Producers of Mexico.

Active in the National Conference of Christians and Jews, Mr. Chapman also had served for a number of years on the board of directors of Howard University.

He also had served on the board of trustees of the American Home Library Foundation and for may years was general counsel to the Pacific Tropical Botanic Garden in Hawaii. He was a charter member of the American Legion.

He is survived by his wife, Ann, of the home; a son, James, of Washington, and a sister, Bessie Arrington, of Richmond, Va.

The family suggests tht expressions of sympathy be in the form of contributions to Ferrum College, Ferrum, Va., or Denver University Law School.