Patients on oral anticancer agent (OAA) therapies have the autonomy to manage their cancer treatments in home settings. However, patients may not have adequate knowledge, confidence, or ability to effectively manage OAA-related consequences, which can significantly impact their treatment and health outcomes. This study aims to identify the associations between medication beliefs, patient activation, and self-rated health (SRH) among oncology patients taking OAAs and explore the potential mediation effects of patient activation on the relationship between medication beliefs and SRH. A secondary data analysis was conducted on cross-sectional data from 114 patients who were diagnosed with breast, colorectal, lung, or prostate cancer. Patients completed a self-reported survey including items of SRH, Beliefs about Medicines Questionnaire (BMQ), and Patient Activation Measure (PAM-13). Descriptive statistics, bivariate correlation, hierarchical multiple linear regression, and mediation analysis were conducted. The results indicate that patients taking OAAs have ambivalent attitudes toward medication. Both medication necessity (r = - 0.27) and concerns (r = - 0.21) were negatively associated with SRH, while patient activation was positively associated with SRH (r = 0.38). Patient activation had a negative association with medication concerns (r = - 0.36) and fully mediated the relationship between medication concerns and SRH in patients taking OAAs (indirect effect = - 0.154, 95% confidence interval, - 0.276 to - 0.060). The findings highlight the significance of activating patients to better understand and manage their OAAs. It is crucial for oncology professionals to provide multifaceted interventions to promote patient activation with an effort to mitigate the negative impact of medication beliefs on patient-perceived health outcomes.