Objective. We aimed to identify factors associated with treatment failure in patients with acute exacerbation of COPD (AECOPD) admitted to the emergency department observation unit (EDOU). Methods. A retrospective cohort study was conducted between January 1, 2013, and October 31, 2019. The electronic medical records were reviewed of patients with AECOPD admitted to the EDOU. The patients were divided into treatment failure and treatment success groups. Treatment failure was defined as prolonged stay at the EDOU (>48 h) or COPD-related ED revisit (within 72 h) or readmission within 1 month. The two groups were compared and analyzed using univariable and multivariable analyses by logistic regression. Results. Of the 220 patients enrolled, 82 (37.3%) developed treatment failure. Factors associated with treatment failure included arrhythmias (odds ratio [OR] 3.8, 95% confidence interval [CI] 1.04–13.9), diabetic mellitus (OR 2.32, 95% CI 1.09–4.95), long-term oxygen therapy (OR 2.89, 95% CI 1.08–7.72), short-acting beta-agonist use (OR 6.06, 95% CI 1.98–18.62), pneumonia findings on chest X-ray (OR 3.24, 95% CI 1.06–9.95), and ED length of stay less than 4 h (OR 2, 95% CI 1.08–3.73). Conclusion. Arrhythmias, diabetic mellitus, long-term oxygen therapy, short-acting beta-agonist use, pneumonia findings on chest X-ray, and ED length of stay <4 h were the significant factors associated with treatment failure of AECOPD to which physicians at the ED should pay special attention before the admission of patients to the EDOU.