Margaret O'Brien (born November 20, 1973) is a former member of the Michigan Senate and former member of the Michigan House of Representatives. She is a Republican. Her district was based in Kalamazoo, Michigan. While in the House of Representatives, O'Brien twice was unanimously elected as Associate Speaker Pro Tem. She often presided over sessions of the House. In the State Senate, O'Brien was unanimously elected as Assistant President Pro Tem where she again frequently presided over session.