Dr O’Sullivan’s research focuses on the transmission of learning in the early Middle Ages. Her main focus is on early medieval glosses and their importance.

She has published two books on the reception of Late Antique authors: the first on Prudentius (Brill), the second on Martianus Capella in the series Corpus Christianorum Continuatio Mediaevalis (Brepols).

She has also co-edited a book on Martianus Capella and Carolingian Scholarship (Brepols). Her current work investigates the significance of glosses for mainstream cultural and intellectual history.