The Washington Post Co. yesterday announced that Anthony J.F. O'Reilly, chairman, president and chief executive officer of H.J. Heinz Co., and Martin Cohen, who has been vice president-finance and treasurer of The Washington Post Co., were elected to the company's board of directors.

Cohen will remain a vice president of the company with responsibility for newsprint interests, affiliated operations and pension funds.

Alan G. Spoon, who has been vice president-marketing and finance of The Washington Post newspaper, will move to the corporate office as vice president-finance. Leonade D. Jones moves up from assistant treasurer to treasurer.

The election of Cohen and O'Reilly to The Post Co.'s board increases the number of directors from 10 to 12. O'Reilly has served as president and chief executive officer of H.J. Heinz since June 1979 and became chairman earlier this year. Born in Dublin in 1936, O'Reilly also is chairman of Fitzwilton Ltd., chairman and controlling stockholder of Independent Newspapers Ltd., Ireland's largest publishing group, and chairman of Atlantic Resources, an oil and gas exploration group based in Dublin.

O'Reilly, who lives outside Pittsburgh, is a director of Mobil Corp., Allegheny International Inc., Bankers Trust New York Corp. and Bankers Trust Co. He also serves as a partner in Cawley Sheerin Wynne & Co., solicitors of Dublin, and is a member of the supervisory board of the Thyssen-Bornemisza group.

Cohen joined The Post Co. in 1966 as manager, special projects, for the Post and Newsweek. He became controller of the newspaper in 1969 and controller of The Washington Post Co. in 1973. He was elected a vice president in 1974 and vice president-finance and treasurer of The Post Co. in 1975. Previously, Cohen worked for Price Waterhouse and the Treasury Department.

Spoon has been vice president-marketing and finance of The Washington Post newspaper since January 1986. He became vice president-controller of the newspaper in 1984 after joining the company as vice president in 1982. Previously, Spoon was vice president and partner of Boston Consulting Group.

Jones has been assistant treasurer of The Post Co. since November 1985. She was vice president-business affairs for Post-Newsweek Stations from 1984 to 1985 and director of financial services for Post-Newsweek Stations from 1979 to 1984. She joined The Post Co. in 1975 as assistant treasurer. Previously, Jones was a financial analyst with Capital Research Co.

In other announcements yesterday, newspaper President Thomas H. Ferguson announced that Thomas O. Might was named vice president-marketing and production for The Washington Post newspaper. Ferguson said Might will assume Spoon's responsibilities in the marketing, promotion and research areas and will report to him on those matters. Might will continue to report to Don Rice, vice president-operations, on newspaper production matters.

Ferguson said Peggy Schiff will continue to report to him as vice president-controller, while Beth Loker, vice president-advanced systems, will report to Ted Lutz, vice president-business manager.