Abstract Introduction The aims of this study were to determine the associations between psychological wellbeing, and the frequency of tooth brushing and presence of oral ulcers during the COVID-19 pandemic; and to identify the mediating roles of psychological distress (general anxiety and depression) and perceived social support in the paths of observed associations. Methods This cross-sectional study recruited 996 adults in Nigeria between June and August 2020. Data collected through an online survey included outcome variables (decreased frequency of tooth brushing and presence of oral ulcers), explanatory variable (psychological wellbeing), mediators (general anxiety symptoms, depression symptoms and perceived social support) and confounders (age, sex at birth, educational and employment status). Multivariate logistic regression was used to determine the risk indicators for the outcome variables. A path analysis was conducted to identify the indirect effect of mediators on the association between the outcome and explanatory variables. Results Of the 966 respondents, 96 (9.9%) reported decreased tooth-brushing frequency and 129 (13.4%) had oral ulcers during the pandemic. The odds of decreased tooth-brushing during the pandemic decreased as the psychological wellbeing increased (AOR: 0.87; 95% CI: 0.83–0.91; p