Merrill Lynch on Tuesday said that Mr. O’Neal, its embattled chairman and chief executive, is retiring, effective immediately, marking the official announcement of a departure that the Street has been expecting for days. Merrill said it had named a director, Alberto Cribiore, as the securities firm’s interim nonexecutive chairman, but the chief executive post will remain vacant for now.

Mr. Cribiore, a private equity executive, will lead a committee charged with the highly delicate task of finding Mr. O’Neal’s successor.

Mr. O’Neal’s tenure as Merrill’s chief executive came to an abrupt end after the company reported a much bigger-than-expected $8.4 billion write-down in the third quarter. The board’s dissatisfaction with Mr. O’Neal only grew when he held unauthorized talks about a possible merger with Wachovia.

Merrill said that Mr. Cribiore, a director since 2003 and a former president of Clayton Dubilier & Rice, will chair a search committee that will look for a new chief executive “from within and outside” the company. Mr. Cribiore is a managing partner and founder of Brera Capital, a private equity firm.

Mr. Cribiore, a native of Milan, Italy, was the first director Mr. O’Neal added to the Merrill board after he became chief executive in late 2002. The two got to know each other while Mr. O’Neal was a high-yield debt banker and Mr. Cribiore was an executive at Clayton Dubilier & Rice.

In the late 1990s, Mr. Cribiore came close to persuading Mr. O’Neal to join Brera Capital, the private equity firm he founded in 1997.

“We would like to thank Stan for the contribution he has made leading a major transformation of Merrill Lynch into a global and diversified company with enormous potential ahead of it,” Mr. Cribiore said in a statement.

Ahmass Fakahany and Gregory Fleming will remain Merrill’s co-presidents and chief operating officers. Mr. Fakahany, who is seen as a close ally of Mr. O’Neal’s, will still lead global support, finance and human resources, while Mr. Fleming will remain the head of investment-banking operations.

But the question of who will be Merrill’s next chief remained unanswered Tuesday. Since this weekend, reports have named a plethora of possible candidates both outside of Merrill — including BlackRock’s head, Laurence D. Fink, and the NYSE Euronext chief executive, John Thain, — and inside of the firm. Those include Robert McCann, the head of the firm’s brokerage services, and Mr. Fleming.

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