Two dogs were presented to the emergency room for suspected carbon monoxide toxicity associated with a furnace malfunction. Both dogs exhibited signs of impaired ambulation, abnormal mentation, and respiratory distress. Arterial co-oximetry confirmed elevated carboxyhaemoglobin levels (27.6% and 23.6%). Initial treatment consisted of bilateral nasal oxygen administration. Due to progression of clinical signs, dog 1 was mechanically ventilated and dog 2 was administered high flow nasal oxygen (HFNO). After approximately 3 hours of treatment, both dogs had improved carboxyhaemoglobin levels of 4.9% and 5.2%, respectively, and had significant improvement of reported clinical signs. The calculated half-life of carboxyhaemoglobin in dog 1 with mechanical ventilation was 150 minutes, and in dog 2 was 167 minutes with HFNO. Both dogs survived to discharge and no long-term complications were reported. This is the first described case using HFNO as a treatment option for carbon monoxide toxicity in dogs.