Osteosarcoma is the most common primary malignant bone tumor, predominantly affecting adolescents and young adults. Survival outcomes vary widely, and previous studies have suggested differences in patient characteristics and disease patterns across populations; however, population-based evidence from Taiwan remains limited. In this nationwide retrospective cohort study, data from the Taiwan Cancer Registry were analyzed to evaluate overall survival (OS) and prognostic factors among patients diagnosed between 2013 and 2022. A total of 371 patients were included, with a mean follow-up of 4.6 years. Kaplan–Meier analyses demonstrated significant differences in overall survival according to tumor stage, primary tumor site, and histological grade (all log-rank p < 0.05). In multivariable Cox proportional hazards analyses, older age groups were associated with an increased risk of death, while high-stage disease (AJCC stage III–IV) was associated with a significantly higher risk of mortality compared with low-stage disease (HR = 1.94, 95% CI = 1.30–2.90). In contrast, low-grade histology (HR = 0.31, 95% CI = 0.14–0.71) and limb salvage surgery (HR = 0.38, 95% CI = 0.23–0.64) were associated with improved survival. These findings provide population-based evidence on prognostic factors for osteosarcoma in Taiwan and highlight the associations of age, disease stage, tumor biology, and surgical management with overall survival.