Background: The Health Action Process Approach (HAPA) model addresses health behaviours, but it has never been applied to model adolescents’ oral hygiene behaviour during fixed orthodontic treatment. Aim: This study aimed to apply the HAPA model to explain adolescents’ oral hygiene behaviour and dental plaque during orthodontic treatment with fixed appliances. Methods: In this cross-sectional study, 116 adolescents with fixed appliances from an orthodontic clinic situated in Almere (the Netherlands) completed a questionnaire assessing oral health behaviours and the psychosocial factors of the HAPA model. Linear regression analyses were performed to examine the factors associated with dental plaque, toothbrushing, and the use of a proxy brush. Results: Stepwise regression analysis showed that lower amounts of plaque were significantly associated with higher frequency of the use of a proxy brush (R2 = 45%), higher intention of the use of a proxy brush (R2 = 5%), female gender (R2 = 2%), and older age (R2 = 2%). The multiple regression analyses revealed that higher action self-efficacy, intention, maintenance self-efficacy, and a higher education were significantly associated with the use of a proxy brush (R2 = 45%). Conclusion: Decreased levels of dental plaque are mainly associated with increased use of a proxy brush that is subsequently associated with a higher intention and self-efficacy to use the proxy brush.