Lawyers for Mobil Oil Corp. President William P. Tavoulareas and his son Peter virtually completed their $50 million libel case against The Washington Post yesterday after presenting testimony from an early source for the first of two disputed 1979 stories and an editor who supervised its final drafting.

The newspaper's lawyers are expected to start presenting today their version of the events surrounding the Nov. 30 and Dec. 1, 1979, articles, which the Tavoulareases allege were libelous. The stories described how the elder Tavoulareas helped create the Atlas Maritime Co., a London-based firm in which the younger Tavoulareas has a financial interest and which manages the shipping operations for the Saudi Maritime Co. (Samarco), in which Mobil is a partner.

Testimony during the 12-day-old trial before a six-member federal court jury has shown that the elder Tavoulareas knew about Atlas' business affairs, but he and his son both denied that the Mobil executive pressured George Comnas, Atlas' one-time managing director, into accepting Peter Tavoulareas as a partner. Comnas was one of Post reporter Patrick Tyler's key sources for the Nov. 30 story that said the elder Tavoulareas "set up" his son in Atlas.

Dr. Philip Piro, a Baltimore eye surgeon who was married to Tavoulareas' daughter Patrice for four years before an acrimonious 1980 divorce, testified yesterday that he met with Tyler and special correspondent Sandy Golden a month before the newspaper articles were published and described some of the family's business affairs.

Piro, who has been sued by the Tavoulareases for $20 million in a companion case being heard along with the suit against the newspaper, testified that as he spent more and more time with the Tavoulareas family he "became aware of Peter's and William's business closeness." He said he "was more or less asked to tacitly approve."

But Piro, 30, said that he saw "possible tremendous conflict of interest to a large corporation" in the elder Tavoulareas' dealings with Atlas matters.

Piro, under direct questioning by Edwin David Robertson, one of the Tavoulareases' team of lawyers, said he could not recall many of the specific details of his restaurant meeting with Tyler and Golden. But when Robertson read portions of Piro's earlier sworn deposition testimony to him, the physician acknowledged that he had often answered the same questions more precisely with "yes" and "no" answers.

"I did not commit that dialogue to memory," Piro said of his conversation with Tyler and Golden.

Piro said that as his marriage deteriorated, the elder Tavoulareas recommended "every day" to his daughter that she divorce him. The former Tavoulareas son-in-law said the Mobil chief was "vividly vulgar and abusive" during one fight Piro had with his wife.

Piro said he distinctly remembered a September 1977 comment the elder Tavoulareas made to him about his business dealings with his son Peter. Piro quoted the Mobil president as saying, "I only gave him a small nudge to get him along." Tyler quoted Piro in a similar vein in the Nov. 30 story, although Tavoulareas has denied making the remark.

In other testimony yesterday, William Greider, a former Post assistant managing editor for national news, said he made a "fairly careful line by line" review of Tyler's Nov. 30 story before it was published.

"I asked him all through his piece who his sources were," Greider said. "He satisfied me the sources were solid and that the story was solid."

U.S. District Judge Oliver Gasch, who is presiding at the trial, rescinded yesterday part of an earlier order that allowed Mobil 24 hours to review each day's transcript before it was released to the public to determine if parts of it ought to be sealed to protect Mobil's trade secrets.

But Gasch left open a decision on a request by the Reporters Committee for Freedom of the Press and four individual reporters to unseal various Mobil documents that have been submitted in evidence at the trial. Gasch said the oil company should have a chance to support its claim for continued confidentiality and gave Mobil until 30 days after the trial to do so.