A 75-year-old man with known chronic obstructive pulmonary disease (COPD) is brought to the Emergency Department with increased breathlessness, wheeze, and a change in sputum colour to green. His oxygen saturation is 89% on room air. Arterial blood gas (ABG) on room air shows: pH 7.32, PaCO2 7.8 kPa, PaO2 7.0 kPa, HCO3- 30 mmol/L. According to NICE guidelines, what is the most appropriate initial oxygen therapy?