Interstate Commerce Commission member A. Daniel O'Neal and Quaker Oats executive Sam Hall Flint are leading candidates to become chairman of the regulatory agency, government sources said yesterday.

O'Neal, a commissioner since 1973 and former transportation counsel to the Senate Commerce Committee, was given the inside track by some officials - particularly if President Carter accepts recommendations by his advisers to reduce the size the nation's oldest economic regulatory body.

The ICC which regulates interstate trucking, railroads, oil pipelines, buses and inland barges, has 11 commissioners - the largest number among major agencies.

Carter's aides have suggested a reduction in size to seven or nine members and there are two vacancies at the moment. Since one method of reducing size temporarily would be not to add any more members, sources said, the 40-year-old O'Neal would appear the most likely choice. A permanent reduction would have to be a proposed as a formal reorganization by the President, which Congress could veto.

Terms of two other members - Virginia Mae Brown and Dale W. Hardin - expire at the end of this year.

Flint, who will be 57 in March, has been vice president for corporate operations of the Quaker Oats Co., of Chicago, since 1973. He is a native of Mt. Airy, Ga., was director of transportation for the Georgia Public Utilities Commission from 1946 to 1953 and is a former president of the ICC Practitioners Association and the National Freight Traffic Association.

O'Neal is a native of Washington State with strong support on Capitol Hill. A former legislative aide to Senate Commerce Committee chairman Warren Magnuson (D-Wash.), O'Neal directed a two-year study by the committee into the Penn Central Railroad's collapse.

One factor that could hurt Flint's chances, sources said, is his Georgia birthplace since membership on the agency generally is sprinkled throughout various parts of the country. A current members is Georgian Rupert L. Murphy, a commissioner since 1955 whose term expires at the end of 1978.

Flint, whose opinions on transportation were sought by Carter during last year's election campaign, previously has been active in lobbying groups that favor less ICC control over rates and entry into the trucking business. O'Neal spoke out recently in favor of more rapid reform at the ICC.