The Wikipedia summary, here -- very short -- is pretty good, with the Tax Division experience highlighted:
After working as a law clerk, Oberdorfer went into private practice in Washington D.C. with the firm Paul, Weiss, Wharton & Garrison as a tax attorney until his friend Deputy Attorney General Byron White asked him to join the Robert Kennedy Justice Department in 1961. He was hired as Assistant Attorney General, Tax Division but, since the division was largely organized and self-sustaining, he focused his energies on many legal issues, particularly civil rights.
He returned to private practice in 1965 with Wilmer, Cutler, & Pickering. Oberdorfer remained friendly with the Kennedy family and personally represented Jacqueline Kennedy in a 1966-1967 public legal battle with historian William Manchester over the ownership of interview materials and their publication in his book The Death of a President about the John F. Kennedy assassination. In 1968, Oberdorfer was elected co-chairman of Lawyers' Committee for Civil Rights Under Law. He served as president of the District of Columbia Bar Association in 1977-1978. When Griffin Bell became attorney general in 1977, Oberdorfer was considered for the deputy position, but was instead appointed to the United States District Court for the District of Columbia. He has championed opposition to mandatory sentencing policies, especially with respect to drug offenders. He assumed status as a senior judge in 1992. He also taught part-time at Georgetown Law Center from 1993 until his death.Also, the D.C. Bar had a series on Legends in the Law, including one on Judge Oberdorfer, here. I cut and paste the part on his tenure in the tax division (with the predicate part about his entry into the field of tax law; I also highlight some things of interest, at least to me).
BR: How did you enter private practice?
LO: I had an offer from the firm of Paul Weiss Wharton & Garrison. Lloyd K. Garrison, who was serving as special master appointed by the Supreme Court in the case of Georgia v. The Pennsylvania Railroad, came into Black’s chambers one day and invited me to come into the New York office of Paul, Weiss. I didn’t want to go to New York, but they then had a small tax office in Washington headed by Randolph E. Paul who wrote the first scholarly treatise on the federal income tax and the first scholarly treatise on the federal estate tax. At the time Paul and his partners were the ultimate pros in the tax field. I went down to the office to talk to them and they said they do only tax work, which I wasn’t interested in. Paul said, "I turned down around a hundred thousand dollars of non-tax business last year. Why don’t you come down here and take care of the non-tax cases." I took the job and I never saw a non-tax case! By default I became a tax lawyer.