U.S. District Judge Oliver Gasch overturned yesterday a jury's verdict last summer that a former son-in-law of Mobil Oil Corp. president William P. Tavoulareas slandered Tavoulareas in comments about his business dealings.

Gasch threw out the jury's $5,000 damage award against Dr. Philip Piro, who was once married to Tavoulareas' daughter, Patrice, and is now an eye surgeon at a Hollywood hospital.

But the judge let stand the jury's verdict that Piro slandered Tavoulareas' son, Peter, as well as the jury's $1,000 award to him.

The Piro case centers on comments he made in 1979 to Washington Post reporter Patrick Tyler and one-time special correspondent Sandy Golden during an interview. Tyler later wrote a Nov. 30, 1979, article saying that the elder Tavoulareas "set up his son" Peter in 1974 as a partner in a London shipping firm.

The same jury that ruled against Piro also decided that The Post libeled the Mobil executive and awarded him $2,050,000 in damages. But Gasch reversed that ruling earlier this month, saying there was "no evidence to support the jury's verdict." The judge ruled that the elder Tavoulareas did not prove that the newspaper committed "actual malice" in printing the story, that is, knowingly printing a falsehood or showing reckless disregard of the truth.

Tavoulareas has until June 1 to decide whether to appeal Gasch's decision.

Piro described Gasch's decision as "a great victory for freedom of speech. I feel completely vindicated." He said he did not know whether he will appeal the $1,000 judgment or pay it until he discusses the ruling with his attorney.