NEW YORK (AP) _ The chairman and chief executive of The Ogilvy Group, an advertising and marketing company recently acquired by Britain's WPP Group PLC, said he had resigned to take a senior post with American Express Co.

Kenneth Roman, 59, said Monday he was ending a 26-year career at Ogilvy because the job offered by American Express was ''simply too exciting to turn down'' and because the post-buyout transition at Ogilvy had been accomplished.

Roman will become executive vice president corporate affairs and communications at American Express, reporting to James D. Robinson III, chairman and chief executive. American Express is one of Ogilvy's clients.

The post Roman is taking is being vacated by Harry L. Freeman, who said in August that he planned to retire by year's end in the wake of disclosures that an American Express investigation of banker Edmund Safra had grown into an unauthorized campaign to discredit Safra. Freeman was not implicated in the campaign, but he explained in announcing his plans to retire that it occurred because of ''mistakes made on my watch.''

Ogilvy was acquired last summer for $864 million by WPP Group, owner of the J. Walter Thompson advertising agency, in what ranks as the biggest takeover in the ad business.

Roman resisted the initial takeover offer of $45 a share, but gave in after London-based WPP sweetened it for a second time to $54 a share.

In a telelphone interview Monday, Roman said he ''wasn't looking'' for a new job when American Express' Robinson approached him in September about Freeman's soon-to-be-vacated position.

''Leaving the company is a hard decision on my part,'' Roman said. ''I have been running Ogilvy & Mather for five years now, I have just managed it through a takeover, managed it through the transition, through a move into a new building and new headquarters and we have just had a big worldwide management meeting where we laid out a new corporate strategy.''

''Everything is in good shape, and I felt it was time for someone to come in and carry it to the next stage,'' he said.

Roman is not being replaced in his positions at The Ogilvy Group because the holding company's functions have largely been taken over by WPP.

But he is being replaced as chairman and chief executive of Ogilvy & Mather Worldwide, Ogilvy's largest advertising company.

Graham Phillips, 50, was named chairman and chief executive of Ogilvy & Mather Worldwide. He has been with Ogilvy & Mather U.S. since 1965, helped found the agency's Houston office in 1970 and has run offices in Amsterdam, Toronto and New York City. He has been chairman of Ogilvy & Mather U.S. since 1985 and president of Ogilvy & Mather North America since 1987.

Roman said Phillips was ''the logical and appropriate choice'' to succeed him.

Alexander Brody, 56, was named to the newly-created position of president of Ogilvy & Mather Worldwide and chief executive of its international operations.

Brody joined Ogilvy & Mather Worldwide as president-international in 1987 and previously headed DYR Worldwide, a global advertising agency network that is a joint venture of New York's Young & Rubicam Inc. and Dentsu Inc. of Japan.