oxygen sat level not improving

my dad has mild to moderate COPD with few scars on his lungs. Also 3 months ago he is diagnosed with sever sleep apnea and he was recommended bipap therapy at 27/25 cm H2O along with supplemental oxygen 2.0 L/min. He is diabetic and takes blood pressure medication. He is 67 years old. His medications are symbicort,phylline,nifedipine,glimpiride,cozaar.
His Oxygen sat level is normally 82 to 88%. 2 days ago he broke the outlet he had with his humidifier with the oxygen machine. Now he is using the oxygen directly  (without humidifier) with the bipap machine and found that his oxygen level went down significantly like 75%. What is happening? Dose the humidifier plays a role here?
How can we bring the oxygen level up? There is a vent on the mask, dose it taking away the part of the oxygen?
Also with supplemental oxygen he should have higher oxygen level, why is it still below 90%? do u think it is due to not using the machine properly?please help me.