A lawyer for Mobil Oil Corp. president William P. Tavoulareas and his son Peter sharply questioned a Washington Post reporter and an editor yesterday about the details of how they pieced together two allegedly libelous 1979 articles about the Tavoulareases' business dealings.

John J. Walsh, one of the Tavoulareases' team of lawyers, paid particular attention to whether the elder Tavoulareas "set up" his son in a London-based shipping management firm, as the first paragraph of the Nov. 30, 1979, story said, and whether the elder Tavoulareas was given an adequate opportunity to respond to a follow-up story the next day before it was published.

Patrick Tyler, The Post reporter who wrote both stories, conceded that the phrase "set up" was not contained in various source materials that he used in writing the lead paragraph of an 80-inch story. The story detailed the creation of the younger Tavoulareas' firm, Atlas Maritime Co., which manages the shipping operations of Saudi Maritime Co. (Samarco), in which Mobil is a partner.

But Tyler, completing about 16 hours of testimony during the 15-day-old trial, said that the phrase "is a common word used by many people." At another point, he said the words were a "simple way of describing how something came to be . . . the simple involvement of William Tavoulareas" in the creation of his son's company.

But Walsh was not satisfied with the reporter's explanation and pressed Tyler to admit that the phrase also could be used in an uncomplimentary fashion.

"Many words have many meanings," Tyler finally said.

"I think someone else will have to determine what it means," Walsh retorted, referring to the six members of the federal court jury hearing the $50-million libel case.

Walsh also questioned Tyler and Bob Woodward, a Post assistant managing editor who did some early reporting on the Tavoulareas story in 1976 and later had a hand in editing Tyler's work, about the fact that no one at the newspaper called Mobil for a response to allegations in the Dec. 1 story before it was published. The Dec. 1 article did contain Mobil's response to the previous day's story.

Tyler's second story about the Tavoulareases quoted a letter written by Rep. John D. Dingell (D-Mich.) to the Securities and Exchange Commission saying that the elder Tavoulareas may have given "false and misleading" statements to federal investigators who questioned him in 1977 about his involvement in his son's firm.

Tyler testified that he had repeatedly asked for Mobil comment on a variety of issues while reporting the two stories and for a chance to interview top Mobil officials, including Tavoulareas. But he conceded that he did not tell Mobil on Nov. 30 that his next day's story would contain Dingell's allegations.

Woodward testified that he originally was surprised to learn that Tyler had not given the elder Tavoulareas a chance to respond specifically to the Dingell allegations.

"We'd been to the well 50 times," Woodward said. "I think he should have gone the 51st time."

Later, however, Woodward said that, "on reevaluation, I'm not particularly surprised" that Tyler did not place the call. "He made many efforts."

The jury heard one other witness yesterday: Herman F. Hoffmann, a former Mobil vice president who now is president of the Ryder Truck Rental firm. Hoffmann described Tavoulareas, his former boss, as a "warm, outgoing, very generous individual" and not at all vindictive, as another former Mobil official had testified on Thursday.

Hoffmann later said that he had been flown to the trial from Indianapolis on a Mobil jet, but added that he expects the Tavoulareases will pay for the flight.