O'Malley diangkat jadi coadjutor uskup diocese of Saint Thomas dina 30 Méi 1984, ku Paus Yohanes Paulus II. Anjeunna narima consecration episcopal na dina 2 Agustus di handap ku Bishop Edward John Harper, jeung Archbishop James Hickey jeung Bishop Eugene Marino porsi salaku co-consecrators.
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