A 42-year-old woman with acute pneumonia (right lower lobe consolidation on CXR) presents with SpO₂ 88% on room air and dyspnea. Arterial blood gas shows PaO₂ 65 mmHg, PaCO₂ 38 mmHg, pH 7.40. A-a gradient is 42 mmHg. She is alert, respiratory rate 24/min, and hemodynamically stable. What is the most appropriate immediate management?