A former Ohio congressman is under investigation by a federal grand jury here on charges that he purchased at least $10,000 worth of merchandise on a gasoline credit card that was provided to him by the president of an Ohio oil firm.

Former representative Charles J. Carney (D-Ohio), who served four terms in Congress before being defeated in 1978, was given the card in 1969 when he was minority leader of the Ohio Senate because he was in a position to "do . . . favors" for the oil company president, according to documents filed in court here yesterday.

He contined to use the card for at least 10 years -- including the whole time he was in Congress -- because he was still in a position to give a favorable treatment on unspecified issues to the company president, the documents continued.

The investigation of Carney came to light yesterday when the oil firm president, William G. Lyden Jr., entered a guilty plea here to one count of offering gifts to Carney in the form of paying the $10,000 worth of charges on the card.

According to the plea agreement, Lyden -- president of the Lyden Oil Co.

in Youngstown, Ohio and former president of the Washington-based National Oil Jobbers Council -- will cooperate in the ongoing Carney investigation.

Lyden was released on his personal recognizance, and the Justice Department said it would recommend that he be placed on probation. U.S. District Court Senior Judge Howard F. Corcoran, who took Lyden's plea, said he was not bound by that recommendation, however.

Justice Department attorney Andrew J. Reich filed with the court an outline of the charges against Lyden that said Carney was first given the Amoco credit card with the agreement that the charges would be paid by Lyden's company.

"After Mr. Carney became a United States congressman, Mr. Lyden continued to pay for Mr. Carney's charges on the credit card because of the favors Mr. Carney could do far Mr. Lyden," Reich said in the statement.

Lyden in turn authorized the payments "for and because of official acts performed and to be performed by Mr. Carney" as a congressman, Reich added.

On at least three occasions when Carney was in office, Lyden sent him renewed credit cards, according to Justice Department investigators. In addition, Lyden "requested various official favors" from Carney while he was a congressman, investigators said.

Oil Express, a weekly newsletter published in Washington that serves petroleum marketers, reported earlier this week that investigators are seeking to tie the credit card payments to a bill that Carney introduced concerning oil jobbers in 1976. Ironically, the news letter reported, the Carney legislation -- which did not pass -- would not have directly affected Lyden's firm because it was too large.

Various staff members of the National Oil Jobbers Council have been questioned in the investigation but are not viewed as possible defendants, investigators said yesterday.

The executive vice president of the council, Robert W. Sullivan, said yesterday the circumstances of Lyden's plea are "most regrettable and unfortunate" but in no way involve the council or any of its activities.

The council is the national organization representing the nation's heating oil and motor fuel marketers and distributors. Lyden was president in 1978.

Carney received considerable publicity in 1977 while he was a congressman for admitting that he had taken more than 60,000 free books from the Library of Congress. He told a newspaper reporter at the time that "I never took any money or stole any money. If you want to call them few books stealing, then I stole."