Aine O'Brien is the former head teacher of Fowlmere Manor and former co head teacher of Elmsmere Manor. In Audition, when her and Crispin Bell are reviewed by the governors, Aine O'Brien is fired, and Crispin Bell is demoted to a teacher, and the former deputy head of Elmsbury Academy, Anita Andress, comes to Elmsmere and is promoted to headteacher.
- Mrs O'Brien is shown to have a strong rivalry with Crispin Bell. In Cross Country, when they announce the Elmsmere status, they constantly are seen to interrupt each other in the speech, and also Mrs O'Brien encourages Dexter to choose a Fowlmere pupil as the winner of the anthem competition as it will make her look good to the governors when they pick a new head.
- Mrs O'Brien seems to enjoy horse-riding, as in Merger a picture of her and a horse is shown in her office.