慈幼會
In 1995 Oster joined the Salesians of Don Bosco and spent a year in the novitiate in Jünkerath. From 1996 till 2000 he studied at the College of Philosophy and Theology in Benediktbeuern. On July 24, 1999 he professed his perpetual vows and thus formally became a member of Salesians of Don Bosco. He was ordained to the priesthood by Viktor Josef Dammertz, the bishop of Augsburg, on 24 June 2001.