Nosenko, who was controversially detained for three years by the CIA, was 81 and had suffered a long, unspecified illness, the report said.
Nosenko told the CIA when he defected in 1964 that Oswald, accused of killing President John F. Kennedy a year earlier, was not a Soviet agent and had been viewed by the KGB as too unstable mentally to be of assistance. Nosenko, a lieutenant colonel in the KGB, had personally interviewed Oswald while the American had lived in the Soviet Union before the assassination.
Senior CIA officials, however, expressed doubt that Nosenko was a real defector and ordered him put in prison.
The problem was that Nosenko had admitted to making false statements when he first contacted CIA agents in 1962 in Geneva. He had offered his services for just $200, saying he had borrowed KGB funds to buy alcohol and was desperate to pay off the debt. It later turned out that he had not incurred any debt, but he did enjoy drinking, The Washington Post said.
Nosenko was imprisoned for three years and faced harsh CIA interrogations during that period. He was freed in 1967 after passing multiple lie detector tests, and he was declared a legitimate defector in 1969.
Nosenko later became a consultant to the CIA, which gave him a new identity and a home in an undisclosed location in a southern U.S. state.
Yury Ivanovich Nosenko was born in 1927 in the Ukrainian town of Nikolayev, and he graduated from the Moscow State Institute of International Relations in 1950. His father rose to the rank of shipbuilding minister under Soviet leader Nikita Khrushchev.