"With the grace of God, I will do my very best to oversee and govern the archdiocese until the appointment of a new archbishop. I ask for your prayers."
With Cardinal O'Brien's resignation from the post, the Archdiocese is known as a "vacant see" and remains so until the appointment of a new archbishop.
The apostolic administrator governs with the authority, obligations and rights of a diocesan bishop until then.
Cardinal O'Brien is now Archbishop Emeritus of St Andrews and Edinburgh and has no role in the governance of the diocese.
A statement from the Scottish Catholic Media Office said: "The Holy Father, Pope Benedict XVI, has appointed the Most Rev Philip Tartaglia, Archbishop of Glasgow, as apostolic administrator of the Archdiocese of St Andrews and Edinburgh. With immediate effect, Archbishop Tartaglia will govern the Archdiocese of St Andrews and Edinburgh until a new Archbishop is appointed."