U.S. District Judge Oliver Gasch yesterday ordered some 3,800 pages of deposition testimony and exhibits unsealed in a libel case filed against The Washington Post by Mobil Oil Corp. president William P. Tavoulareas, but then granted a seven-day delay of his order so Mobil could appeal the decision.

The pages of testimony and other documents dealt largely with details of Mobil's shipping and commercial operations that the oil company contends could be helpful to its competitors. Lawyers for The Post have argued that sealing the material from public view infringes on the public's right to know about a public trial.

Gasch recently reversed a jury's verdict last year that The Post libeled the Mobil chief in a 1979 article about his and his son's business dealings. Tavoulareas has appealed Gasch's decision.

In ruling on The Post's motion to have the records unsealed now that the trial is over, Gasch said the depositions have all been filed with the court and became part of the public record. He said in his ruling that Mobil has argued that the transcripts generally contain confidential information but it would be too time-consuming and expensive to give particulars.

"These general assertions are not persuasive," the judge ruled.

At a hearing requested by Mobil following Gasch's order, Mobil attorney Judah Best contended Gasch should postpone the effective date of his order until Mobil can appeal. Otherwise, he said, the appeal would be moot.

"I asked you to show why it should not be made public, and I got very little help," Gasch said. "I'll let you make the argument in the Court of Appeals."

Anthony C. Epstein, a lawyer for the Reporters Committee for Freedom of the Press, which joined with The Post in asking Gasch to unseal the records, contended at the hearing that most of the information in the records had already been made public during the trial--a point Best disputed.

The documents remained sealed in a vault at U.S. District Court during yesterday's proceedings.