Oscar R. Ewing, 90, one of the main authors of the Fair Deal program of President Harry S. Truman, died of apparent pneumonia Tuesday at his home in Chapel Hill, N.C. He had ischemia, a circulatory disorder.

Mr. Ewing was an attorney by profession, a social reformer by avocation and a lifelong Democrat. From 1947 to 1952 he was administrator of the Federal Security Agency, the predecessor of the old Department of Health, Education and Welfare.

Admirers hailed his efforts at the FSA to promote civil rights, extend federal welfare programs and broaden the coverage of the Social Security system. Critics decried what they regarded as his high-handed ways in the bureaucracy and such practices as having a chef from St. Elizabeth Hospital, which was administered by the FSA, prepare meals in his official private dining room.

Mr. Ewing's power -- and his accomplishments in government and politics -- derived from his close relationship with Mr. Truman. He is credited with having started a series of quiet and unpublicized meetings in his apartment at the old Wardman Park Hotel at which the basic policies of the Fair Deal were formulated. The Fair Deal brought coherence to the policies of the Truman administration and were a major factor in his upset victory over Gov. Thomas E. Dewey of New York in the 1948 presidential election. o

Those who met at Mr. Ewing's behest included Clark M. Clifford, then Mr. Truman's special counsel and the last secretary of Defense in the Johnson administration; Leon Keyserling, a member of the Council of Economic Advisers and an advocate of planned economic growth; C. Girard (Jebby) Davidson, assistant secretary of the Interior; David A. Morse, assistant secretary of Labor, and Charles S. Murphy, an administrative assistant to Mr. Truman.

The gatherings began shortly after the Republican sweep in the Congressional elections of 1946. Among the policies the group advocated to Mr. Truman were the recognition of the State of Israel in 1948, Mr. Truman's veto of the Taft-Hartley Labor Act, and the underpinnings of the "welfare state."

The New Deal of President Franklin D. Roosevelt focused on the nation's economic recovery from the Great Depression. It placed particular emphasis on the plight of the poor. World War II largely cured these ills. The Fair Deal was designed to ensure continued economic growth and a more equitable distribution of wealth. The mechanism for this, as it emerged in the Fair Deal, was a series of government programs designed to soften and control the laws of supply and demand that prevailed in an unfettered market economy.

Mr. Ewing and Clifford were the two men who were mainly responsible for running Mr. Truman's election campaign -- including his famous "whistle-stop" tour -- from Washington.

In addition to running the Federal Security Agency, Mr. Ewing worked hard to have the organization raised to the level of a Cabinet department. This did not happen until the first Eisenhower administration took office in 1953 and the Department of Health, Education and Welfare was organized.

Mr. Ewing was responsible for opening the old Gallinger Hospital, now D.C. General, to black doctors. He announced that blacks would be eligible for internships and residencies at the facility.

He also was a strong advocate of national health insurance.

"Our agency, more than any other in the federal governmnet, deals with human values," Mr. Ewing once said of the FSA. "The degree to which we can give all citizens equal access to the basic human needs is the measure of practical democracy."

Mr. Ewing returned to the private practice of law after leaving the government in 1952. In 1960, he moved to Chapel Hill. From 1963 to 1967, he was chairman of the Research Triangle Regional Planning Commission. He was a director of the Research Traingle Foundation from 1960 until his death.

Oscar Ross Ewing was born in Greensburg, Ind., on March 8, 1889, the son of George McClellan Ewing and Nettie Ross Ewing.

He graduated from Indiana University in 1910 and from the Harvard Law School in 1913. He served briefly as an instructor at the University of Iowa Law School and then joined a firm in Indianapolis. He served in the Army during World War I and left the service with the rank of captain.

In 1920, he joined the New York firm of Hughes, Schurman and Dwight and remained with it until 1937. In that year, he was a founder of the firm of Hughes, Hubbard and Ewing. He remained with that firm until 1947, when he became a special assistant to the U.S. Attorney General and then administrator of the FSA.

By then he had long been involved in Democratic politics. He was assistant chairman of the Democratic National Committee from 1940 to 1942 and vice chairman from 1942 until 1947.

In 1942, he was a special U.S. prosecutor who won the conviction of William Dudley Pelley, the leader of the Silver Shirts, on charges of sedition. In 1947, he won the convictions of Douglas Chandler and Robert Best on charges of treason. Both men had broadcast for the Nazis during World War II.

Mr. Ewing's first wife, the former Helen E. Dennis, whom he married on Nov. 4, 1915, died in 1953.

Survivors include his wife, Mary Whiting MacKay Thomas Ewing whom he married on Oct 12, 1955, of Chapel Hill; two sons by his first marriage, James D., of Keene, N.H. five grandchildren and five great-grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Oscar R. Ewing Loan Fund, Harvard Law School, Cambridge, Mass.