John O. Koehler, selected by President Reagan to succeed Patrick J. Buchanan as White House communications director, was a member of a Nazi youth group as a 10-year-old living in Germany, NBC News reported last night.

Koehler, 56, told NBC that he left the group after six months. The White House declined comment on the report.

Koehler, who retired in 1985 as assistant general manager and managing director of the World Services Division of the Associated Press, is an adviser to Charles Z. Wick, director of the U.S. Information Agency.

In a brief telephone interview last night with The Washington Post, Koehler said he had been a member of the youth group known as Jungvolk. "If you lived in Germany at that time and were of a certain age, you had something to do with the party. Do you really begin to think at that age?

"Having been a newspaperman in this country for more than 30 years, I think this is a black day in journalism," Koehler said.

Koehler, who was born in Dresden, noted that he had served in the U.S. Army and Army Reserve from 1954 to 1967. He said he had listed his membership in the Jungvolk when he had his security clearance updated for the White House post.

During his career at the AP, Koehler worked as a corresponndent in Berlin and in Bonn, as bureau chief in Newark, N.J., and as an executive at the news service's New York headquarters. Since his retirement, he has been president of Koehler International Ltd., a consulting firm specializing in public affairs and communication, in Stamford, Conn.