A 72-year-old man with a 45 pack-year smoking history and moderate COPD (FEV₁ 55% predicted) presents with increased cough, purulent sputum and worsening dyspnoea over three days. He is afebrile but tachypnoeic. Oxygen saturation is 89% on room air (baseline 93%). Chest auscultation reveals diffuse wheeze and crackles. There are no signs of consolidation on examination. What is the most appropriate initial management in the emergency department?