U.S. District Court Judge Oliver Gasch, a member of the federal bench in Washington for 16 years, plans to assume the semiretired status of senior judge on Dec. 1, giving President Reagan his first opportunity to nominate a candidate to the court here.

Gasch, 75, said in a brief announcement distributed to his colleagues on the court that he had formally notified the White House of his decision Thursday.

"I figure that 75 is as good a time as any to step down," Gasch said during an interview in his chambers yesterday. Gasch, well known for his long working hours and his prolific but concise judicial decisions, said that while he hopes for extra vacation time, he expects he will still maintain a substantial caseload.

Gasch, a native Washingtonian, began his legal practice in the District 50 years ago, working with the old Capitol Transit Co. In 1937, he took a job as a city attorney in the D.C. Corporation Counsel's office, where he was chief trial attorney. During World War II, Gasch served in the U.S. Army's Judge Advocate General's office and later took reserve status as a lieutenant colonel.

In 1953, Gasch became the principal assistant U.S. Attorney, the No. 2 man in the federal prosecutor's office here. In 1956, Gasch was appointed U.S. Attorney and held the top prosecutor's job until 1961. He then entered private law practice and served as a president of the D.C. Bar Association.

President Johnson appointed Gasch to the District Court in Washington in August 1965. He was sworn in as a federal judge on the same day as William B. Bryant, who yesterday stepped down as chief judge of that court. Bryant, 70, who is succeeded as chief judge by Judge John Lewis Smith Jr., said he plans to maintain "active" status as a member of the 15-judge trial court.

While on the federal bench, Gasch presided over the criminal trial of former Senate secretary Bobby Baker, who was eventually convicted of income tax evasion, and sentenced by Gasch to serve one to three years in prison. In recent years, Gasch ruled that President Carter had violated the Constitution when he unilaterally terminated the mutual defense treaty between the U.S. and Taiwan, a decision that was reversed in the federal appeals court. He was also the judge at the trial of former representative Charles C. Diggs Jr. (D-Mich.), convicted in 1978 of mail fraud and other charges for illegally diverting more than $60,000 of his congressional payroll to his own use. Diggs was sentenced by Gasch to serve up to three years in prison.