Dec 9 (Reuters) - Drugmaker Gilead Sciences Inc will name Roche Holding AG executive Daniel O?Day as its new chief executive officer, according to a source familiar with the matter. The announcement of O?Day?s hiring could come as early as Monday, the source said. It is not clear when O?Day, currently the CEO of Roche Pharmaceuticals, will start at Gilead. Gilead said in July that current CEO John Milligan would step down as soon as the end of the year.