A 62-year-old man with chronic obstructive pulmonary disease (COPD) and no prior pneumococcal vaccination presents with acute onset fever (40.2°C), productive cough with rusty sputum, dyspnea, and pleuritic chest pain. Chest X-ray shows left lower lobe consolidation. Sputum Gram stain reveals Gram-positive diplococci. Blood culture is pending. Oxygen saturation is 88% on room air. What is the most appropriate immediate next step in management?