A 55-year-old man with long-standing smoking history presents with sudden onset pleuritic chest pain and dyspnoea. Chest X-ray shows a 3 cm rim of air between the lung margin and chest wall at the right apex but no mediastinal shift. He is haemodynamically stable with SpO2 96% on air. According to standard management of primary spontaneous pneumothorax, what is the most appropriate initial management?