A fall in oxygen saturation to 70% is a most serious problem and
oxygen alone is almost certainly not adequate
therapy. You should definitely be evaluated with a trial of CPAP. There could well be a secondary benefit to optimum CPAP usage in that there is medical evidence to suggest that nocturnal asthma may subside significantly when sleep apnea is controlled.
Neither oxygen nor inhaled medication, either alone or together, is likely to provide good asthma control.