It means he's stepped down from from the rights and duties of a cardinal, following accusations of inappropriate sexual behaviour towards priests in the 1980s.
Cardinal O'Brien stepped down as Archbishop of St Andrews and Edinburgh in 2013, issuing an apology at the time, saying "there have been times that my sexual conduct has fallen below the standards expected of me".
Pope Francis is understood to have come to his decision after sending a personal envoy, Archbishop Charles Scicluna, on a fact-finding mission to Scotland last year.
Cardinal O'Brien's resignation followed a private discussion with Pope Francis which was preceded by a period of prayer and penance in order to allow the Cardinal to reflect upon his misconduct.
In what is being viewed by the church as an "unprecedented" action, Cardinal O'Brien has removed himself from the key duties of a Cardinal, including, the election of any future Pope and the assistance of the Holy Father in the governance of the Universal Church.
Cardinal O'Brien will also be reduced to a "strictly private" life with no further participation in any public, religious or civil events.
In a statement, his successor Archbishop Leo Cushley of St Andrews & Edinburgh said; "As most people are aware, Pope Francis is a good and prayerful man whose character embodies justice and mercy. I am confident therefore that the decision of the Holy Father is fair, equitable and proportionate,"
"Cardinal O'Brien's behaviour distressed many, demoralised faithful Catholics and made the Church less credible to those who are not Catholic. I therefore acknowledge and welcome his apology to those affected by his behaviour and also to the people of Scotland, especially the Catholic community."
