Hello e mahboob,
Thank you for using JustAnswer Health for your question.
I think your main concern is whether his feet and face swelling is due from the lack of oxygen? The answer, is no, it is not at least directly related to this. It is due to a fluid imbalance. In a person with COPD it would be due to right sided heart failure, or congestive heart failure. Having improved oxygenation now may very well help, as this will make his heart not have to work quite as hard as it does when the oxygen saturation of the blood is low. Also, he could use an evaluation with a pulmonolgist or cardiologist because sometimes a medication that increases the strength of the heart contractions can help to remove the edema, and/or a medication called a diuretic can also help with this. He needs blood testing and physical evaluation to determine which would be the best therapy for him at this time.
I am glad it was discovered that the oxygen mask was not sealing to his face correctly. Keeping the oxygen mask in place will help him.
It also helps to keep the swollen areas elevated as much as possible. His face will naturally be elevated if he is sitting up and this should begin to help. He should elevate his feet at least 3 times per day - up to hip level or heart level will help the most, if possible. Also compression stockings can be worn to help with the leg swelling.
How long this will take to goes away depends on many factors, including whether his electrolyte balance is good and whether or not his heart is pumping strongly. It is important to make sure he does not eat too much salt, and it often helps to make sure he is getting enough potassium. Potassium is best taken from natural sources, such as bananas and other fruits and vegetables.
Ultimately it would be best if he could see a doctor in person so that lab samples can be done and a physical evaluation.
I hope this helps you. If you have questions related to my answer, please reply to this post so that I can respond to you until you are satisfied with the answer.