Just to make sure you understand (and I know that you understand): the oximeter is measuring the oxygen levels in your blood. I assume you used the typical fingertip style. You wore it at night (but not during the day, right?) and it measured your blood oxygen levels as you slept.
Low blood oxygen (saturation) is just one of many symptoms of sleep apnea. It is also the symptom for a LOT of other illnesses, conditions, etc etc. Looking in your throat with a camera is good but does not diagnose sleep apnea. It can point toward the diagnosis (small airway, small mouth, etc) but unless they are looking in there while you are asleep, it does no good in diagnosing sleep apnea. What they ruled out was an obstruction from perhaps a tumor or something else that pressed against your airway or blocked your airway such as nasal polyps. This is how my brother was diagnosed with his. When he had a cold or allergy symptoms, he could breath out through his nose but not breathe in. The polyp enlarged due to the virus/allergies and blocked his nose. The doc used a small camera to see it.
Knowing your blood oxygen levels during sleeping is good but, really, does not indicate sleep apnea. The fact it got better with the machine also does not indicate sleep apnea. It indicates you breathed better, yes, but not because the CPAP kept your airway open.
Diagnosing you due to the single test of the oximeter is rather odd. It is like diagnosing you with MS because you stumble. Or with brain tumor because you have migraines. The data your CPAP collects each night is also rather worthless since there is not anything to compare it to. You cannot say today was warmer than last Tuesday if you don't know the temperature from last Tuesday. You can guess going by memory and deduction but it is not enough.
Personally, I would demand (strongly request) a sleep study. You want to know what your numbers are without the machine and then with. Then you will know if you truly have sleep apnea. AND, better yet, you will know which sleep apnea you have. Obstructive means the throat closes/collapses and blocks the airway. Central means the brain is misinterpreting the CO2 and O2 levels and telling you not to breathe (basically). Some people have both (called mixed sleep apnea). Each type requires different treatments. Each would result in low blood oxygen but, without the sleep test results, you don't know which one and why.