John Thain will become the new chairman and chief executive of Merrill Lynch & Co. Mr. Thain will take over duties at Merrill Lynch effective Dec. 1 2007 taking the place of Stanley O'Neal, who stepped down last month after Merrill Lynch's $8 billion loss on subprime mortgage investments. Wall street investors seemed delighted with the news because at one point the stock was up as much as 7 percent following the news. Merrill Lynch stock finished 1.8 percent higher.