Osoro was born in Castañeda in northern Spain on in 1945. He studied at the Escuela Normal and taught in Santander where its bishop Juan Antonio de Val Gallo ordained him in 1973. He was the vicar general for Santander from 1976 until 1994. He received his episcopal consecration in 1997 when he was named the Bishop of Orense. Osoro became the Archbishop of Oviedo in 2002 and Archbishop of Valencia in 2009. Pope Francis named him as the Archbishop of Madrid in August 2014. Osoro Sierra is considered to be an orthodox prelate known for his pastoral sensitivities. Pope Francis announced that he would elevate Osoro Sierra to the rank of cardinal on 19 November 2016, where he was made Cardinal-Priest of Santa Maria in Trastevere. Francis made him a member of the Congregation for Catholic Education on 23 December 2017. Pope Francis has referred to Osoro Sierra as "Don Carlos the pilgrim", alluding to the cardinal's frequent walks around his archdiocese during which he mingles with the people to get to know them better. He has made religious vocations one of his chief priorities and in Valencia the number of seminarians grew from 51 in 2012 to 61 in 2013 due to his efforts. Upon his arrival in Madrid he planned to launch a diocesan plan for evangelization in order to reach out to those cut off from the Church and to renew vigor in the Christian faith. He invites adolescents to join him at the start of each month for a vigil in which to dialogue with them to understand their needs and concerns and he often celebrates Mass in prisons. Osoro Sierra also visited cloistered convents when he arrived in Madrid. Osoro Sierra was once asked whether he would define himself as a liberal or conservative prelate. But he insisted that he was "a man of the Church" and not one of labels which seek to distract the Church's work. Despite some news reports in April 2016 that Osoro had prohibited Cardinal Gerhard Müller from discussing his book at a Madrid college because Osoro deemed it "against the pope", Osoro welcomed Müller to a discussion of the book on 3 May at the Universidad Francisco de Vitoria, a Roman Catholic university in Madrid. Pope Francis named him a member of the Congregation for the Oriental Churches on 6 August 2019.
