The former US marine lived briefly in the Soviet Union, leading to speculation that he was a Soviet agent - and counter-speculation that he was a convenient patsy for American right-wingers. The files - assembled at the request of the Russian President, Boris Yeltsin - were handed to the US at the weekend during the G8 summit.
They are unlikely to prompt any great revelations. "The Russians opened their files to Norman Mailer and Lawrence Schiller years ago, resulting in Mailer's Oswald biography," said John Locke, an independent researcher on the assasination. "Some details may emerge, but they won't be central." Large parts of the KGB files were revealed in Izvestia several years ago. But they will provide new information on one of the strangest eras in Oswald's life.
Oswald was a source of great fascination for the KGB, which concluded he could not possibly have been the only assassin: he must have been part of a larger conspiracy. But the Izvestia reporter, Sergei Mostovshchikov, said this was partly professional jealousy. They could not understand how a man under their noses for so long and disregarded, had emerged as such a significant figure.
Oswald arrived in the Soviet Union in October 1959, having flown from London to Helsinki. He claimed political asylum and when the authorities tried to throw him out, he slashed his wrists. They let him stay.
He was under heavy surveillance while he lived in Minsk, now the capital of Belarus, because the KGB believed he worked for the other side. They were extremely interested when he bought a rifle in August 1960 and joined a hunting club because they suspected he would use this as an excuse to visit secret facilities while out on "hunting" trips. In fact he sold the rifle shortly afterwards and was a poor marksman.
He had a job as a low-level worker at a radio factory, and in March 1961 he met 19-year-old Marina Prusakova at a dance. They married and had a child.
The KGB was increasingly suspicious that Oswald missed "numerous union meetings and cultural events", and doubted his political commitment.
When he finally returned to America in 1962, disillusioned by his experiences, he said to a neighbour: "You go on building your communism by yourselves. You can't even smile like human beings here."
The authorities decided that he was of no interest as adversary or ally.
Oswald came to the attention of the KGB again when he visited the Soviet embassy in Mexico City, wishing to return to the Soviet Union. He was fed up with being followed and harassed by the FBI, he said. The KGB brushed him off. The next time they heard of the former employee of Department 25 at the Gorizont radio factory was 22 November 1963, the day that Kennedy died.
Izvestia and Mr Mailer say the Oswald files contained no big new secrets. After Belarus became a separate state, the files caused a wrangle with Russia. Now it is up to the US what is released. Whatever is made public, addicts of conspiracy theories will continue to believe still more has been hidden.