Congestive heart failure has many symptoms, but the most dominant, shortness of breath, can be caused by fluid buildup in the lungs. Although inhaled oxygen might temporarily help symptoms of difficulty breathing at night, it might be better to try to get to the root causes. Fluid congestion at night can often be improved over time by reducing sodium intake in the diet or increasing the doses of diuretics.  Medicines that dilate blood vessels can also reduce vascular pressures that contribute to fluid buildup in the lungs. Your father’s doctor could also answer whether there are any treatable conditions that could fundamentally improve your father’s heart function as a pump such as excessive high blood pressure, blocked heart arteries, or impaired heart valves.