A 40-year-old male with rheumatic arthritis and pulmonary interstitial fibrosis is on maintenance steroid therapy. For the past week, he has had a cough with slightly purulent sputum and low-grade fever. Over the last 2 days, dyspnea has progressively worsened. Examination shows marked dyspnea, cyanosis, and widespread crackles in both lower lungs. Heart rate is 110/min with regular rhythm. Arterial blood gas (ABG) on room air reveals pH 7.48, PaCO₂ 30 mmHg, and PaO₂ 45 mmHg. What is the appropriate oxygen therapy for this patient's respiratory failure?