A jury in the bribery trial of former interstate Commerce Commission official Robert L. Oswald was told here yesterday that "he became a private lobbyist" wgile on the public payroll.

Oswald 45, secretary and congressional relations officer of the ICC from 1970 to 1977, is charged with accepting $4,000 and other favors for helping a New York trucking firm obtain agency approval for interstate operation.

"Instead of being a public servant, Mr. Oswald became a private lobbylist for Consolidated Carriers Corp. of New York Xity and Thomas Gambino, one of the owners of Consolidated," Justice Department attorney Judy Dobkin told the jury.

Thomas Gambino is the son of the late Carlo Gambino, who was reputedly the most powerful organized-crime figure in the country.

"The testimony you are to hear concerns corruptions of a government official," the prosecutor told the jury of 10 women and two men during opening statements.

But Oswald's lawyer, Myles Ambrose, said, "The actions that he took are wholly innocent . . . He has never been on conspirator of any sort; he never accepted any bribe."

The government's first major witness, lobbyist Daryl Fleming, testified he was introduced to Thomas Gambino in November 1971 to Tony Grande, a Gambino associate, and that he introduced them to Oswald.

"I explained to Sowald that Gambino had trouble getting authority to operate a truck line," Fleming said.

Fleming said he alter gave Oswald money on three separate occasions out of cash he has been given by Grande, Fleming also said he went to a party at Oswald's apartment and brought a case of Asti Spumanti wine and a case of scotch.

At Christmastime in 1972, Oswald asked him to make reservations for Oswald on an airline to the Bahamas and at a hotel there, Fleming testified.

"Did Oswald tell you who would pay for the trip?" asked government attorney Edward Weiner.

"I think Oswald asked me who would pay," Fleming said. "He said Grande promised him a trip to the islands . . . I told Oswald I was not responsible for the bill. If Grande promised him a trip to the islands, he should talk to Grande."

Fleming testified that Consolidated Carriers received temporary operating authority, then a certificate issued for two years and finally got unlimited authority.