Antimicrobial stewardship programs (ASPs) have gained prominence with increased awareness regarding the importance of appropriate antibiotic use. However, ASP implementation for outpatient antibiotic prescriptions is uncommon, particularly in South Korea. This study aimed to analyse the patterns and appropriateness of outpatient antibiotic prescriptions at a tertiary care hospital in Korea. We analysed the patterns of oral antibiotic prescription from June 1, 2018 to May 31, 2023, at the outpatient department of Seoul National University Bundang Hospital. Appropriateness was assessed for prescriptions issued between May 15 and May 19, 2023. The assessment criteria included: indications for antibiotic use, antibiotic choice, duration, dose, and frequency. Pharmacists and infectious diseases specialists performed evaluations. A total of 7,282,407 outpatient visits were recorded within the 5-year duration. Of these, 243,967 (3.4%) were prescribed oral antibiotics. The frequency of antibiotic prescription was highest in dentistry, dermatology, and urology departments. The most commonly prescribed antibiotics were cephalosporins, penicillins, and sulphonamides. Of the 423 prescriptions evaluated, 289 (68.3%) and 134 (31.7%) were for treatment and prophylaxis, respectively. Inappropriate prescriptions were identified in 28.4% (82/289) and 70.9% (95/134) of the treatment and prophylaxis cases, respectively. The primary reason for inadequacy in both treatment and prophylaxis was inappropriate indications, accounting for 46.3% (38/82) of treatment prescriptions and 96.8% (92/95) of prophylaxis prescriptions. Antibiotics were administered in 3.4% of all outpatient visits, with 28.4% of treatment and 70.9% of prophylactic antibiotics prescribed inappropriately. Proactive and expansive ASP activities by pharmacists should be considered in outpatient settings.