How would I find out if the WWII assignments of a great uncle, whose work in WWII was classified, has been declassified?

We are learning our great uncle, Major Ray Arthur Nichols, who is buried at Arlington, was engaged in classified work during WWII. Wondering how we might find out if that work was ever declassified? We do know he was part of the OSS, and that he flew to China to help secure the release of the Doolittle flyers, who were POWs of the Japanese. Born in 1906; died in 1956.

Thank you!