There are two conflicting theories about what is known as the "Hypoxic Drive", the urge to breathe.  For many years, the accepted knowledge was that a COPD patient who tended to retain Carbon Dioxide (Co2) should never be subjected to increased oxygen flows, as that would destroy their urge to breathe, possibly  to the point of death.  In recent years, the thinking seems to have shifted.  More modern thinking points out that proper saturation is vital for the health of the patient, and that the failure of the Hypoxic Drive due to increased O2 happens only during cases of acute respiratory failure.