First I need to say that everybody (good lungs or not ;D) needs oxygen. It is just that some peoples have lungs (or heart) that simply are not efficient enough any longer to get enough oxygen in their blood without the help of supplemental oxygen. A few people (very few) are given oxygen temporarily after an exacerbation (or flare-up or lung attack - different names for the same thing) because their lungs are recovering from the 'bug'. The point here is that we ALL NEED OXYGEN and are' addicted' to it from the time of our conception! We sometime hear people who think that using supplemental oxygen will make them addicted to it, but this is a total misconception.
People breathe for two critical reasons - to bring in oxygen and to get rid of carbon-dioxide (CO2). Our bodies are truly wonderful things and we have built in 'sensors' that keep it working - sensors that know the level of oxygen in the blood, and sensors that know the level of carbon dioxide in the blood, and sensors that know the pH level (acidity) in the blood and even sensors that detect our bringing air into the lungs.
The sensors that detect our breathing are very powerful - if someone sneaks up behind you and suddenly clamps a hand over your face then you will panic right now, even though you could actually hold your breath for much longer if you chose.
The sensors for carbon dioxide are also very strong - it is primarily these that control how much we breath - how fast and how deep. As the CO2 level in our blood rises these send the message to breath more - if we are not able to breath as they demand (and this happens more ofter as our lungs get worse) then we get the sensation we get that we call shortness of breath (SOB). One interesting feature of these sensors is that what they actually sense is a rise in the level of CO2 and because of this people can become CO2 retainers - they could actually reduce the level of CO2 by concentrating on breathing more, but their bodies are too tired to bother. Ultimately if there is too much CO2 retention then the pH in the blood will change and this is the critical condition known as respiratory acidosis - this is when people are put on a ventilator either by mask or by intubation.
Sorry for all the words here (and I haven't talked about oxygen yet) but it is important to understand some of this before we talk about oxygen.
Our oxygen sensors, compared to the others I have already mentioned, are actually very weak, and oxygen has to drop to a pretty low level before our body pays it any attention. The body's ability to deal with low levels of oxygen include such slow acting responses as instructing the body to produce more red blood cells to carry more oxygen around the body but this response takes a long time.
The level of oxygen in our blood is measured in two different ways. Our doctors prefer to take a blood sample and analyse it in the lab because this way they also get to know the CO2 level and the pH and other interesting things. This blood sample has to be taken from an artery (blood in veins has already lost most of its oxygen) and this arterial blood gas (ABG) sample is normally taken from the artery in the wrist (the one that we feel when we take a pulse), unfortunately getting this sample is sometimes quite painful.
The results that the doctor sees from the lab includes the pressure of the oxygen in the blood and is called the PaO2 and is measured in mm of Hg and is related to how much oxygen the blood can carry. When this level is less than 55 mmHg (at rest) then a study has shown that people live longer when they use supplemental oxygen for more than 15 hours a day. This test was a large and very expensive study with many patients followed over several years. (Thank You Dr. Tom Petty for this study!) The test was very conclusive but really limited, in that it did not look at different levels of blood oxygen before supplementation or at these levels after saturation. There is another study being started in the US now where a higher level of blood oxygen is being studied but the results of this will not be known for quite a few years.
The other way that blood oxygen is measured is much simpler and faster and that is by using a pulse oximeter. These nifty little devices just slip on a fingertip and cost around $100 and give a result in less than a minute even when the patient is active. What they measure is to blood oxygen saturation (SaO2) as a percentage - for people with good lungs this is typically 97% or better. For COPDers this is likely to be lower and a blood oxygen pressure of 55 mmHg is roughly the same as 89% SaO2.
When COPDers are active then their SaO2 will tend to drop and there are different levels that will qualify for oxygen when active. Typically in Canada then supplemental O2 should be prescribed if the SaO2 ever drops below 80%.
The saddest thing is that our doctors tend not to measure O2 saturation levels and seldom check them the patient is active. The 'bottom line' is that you can be very SOB and still have good oxygen levels, and there are people who should be given supplemental oxygen but who have never been tested.
Addendum - After all that, my wife points out that I still haven't answered the original question of why some people with COPD are on oxygen and some are not. And the simple answer is that people get worse gradually and not always in quite the same way - some may find it harder to get rid of the CO2, while others struggle harder to get in enough oxygen. ;D