A C210 pilot reported oxygen system failure resulting in Hypoxia symptoms and diversion to the nearest suitable airport.

Narrative

In this flight; I accidentally became hypoxic due to oxygen equipment failure and my failure to notice it in advance. I began a VFR flight from ZZZ1; planning to climb to 16;500 MSL before crossing the highest terrain on the route. I created a VFR flight plan to ZZZ2 but was unable to raise ZZZ Radio to activate it enroute. I wore an oxygen conserving cannula and used the aircraft's built-in oxygen system; both of which I had used previously this year. I turned the oxygen system on before takeoff; having placed the cannula in proper position on my face for use.As I climbed and reached 16;100 feet; I noticed abnormal feeling mostly described as tingling in my fingers. I had been checking the O2 flow rate and even setting it a bit greater than what was required (a flow rate for 18-19;000 ft). I checked it again and immediately decided to head for lower altitude. My data recorder says I started the descent from 16;800 GPS altitude. I was thinking I may be experiencing carbon monoxide (CO) poisoning or low oxygen saturation; and I began to feel less mentally sharp which was certainly alarming. So I expedited my descent; using the autopilot to maintain the descent; set a target altitude and direct-to nearest and familiar airport; ZZZ. I breathed more deeply and fully; concentrating on not hyperventilating; but even as I descended through 10;000; I did not feel better. I was lightheaded and could tell my decision making was impaired. My concern was that it might worsen; so I continued to actively avoid task saturation by relying on the autopilot to descend to and maintain traffic pattern altitude and to make lateral navigation. I made radio calls for airport advisory rather than uses ASOS but received no responses. I used ForeFlight on my iPad to see what runway was favored by winds to make that choice. I noticed my speech was slurred on the radio; and of course that concerned me too. I continued to experience the tingling; a somewhat rapid breathing; but I had not at all identified the problem. I knew I needed to get on the ground as soon as I could do so safely. I lowered the gear about 5 miles out to help ensure I did not forget it. I flew a left downwind and make radio calls on downwind and short final. I used checklists to avoid missing steps because I could tell I was impaired. Made a nice landing though I think I forgot to deploy flaps. After roll out on RWY XX; I exited at [Taxiway] 1 and did not feel safe to taxi to the ramp. I shut down engine past the hold line at 1 and set the brake. Made a radio call for assistance and got no response. Tried again stating I had a medical emergency; and immediately got a response from the FBO who responded promptly with the fire engine. I got out of the plane to sit on the left main tire in case there was CO in the cabin. The aircraft CO monitor read 15ppm which I believed was not immediately hazardous; but as I continued to feel quite poor and light headed; I had no plan or desire to taxi back to the terminal. My hands and feet were tingling. I also wanted out and to be close to the ground in case I fainted. I wrote a note on my paper notepad describing what felt like and the CO reading in case I could not later describe it.I conversed with the two airport employees who arrived first; wanting to give them symptom description while I could. I also asked then to contact my spouse which they did. Emergency medical personnel arrived shortly thereafter; they found my blood oxygen saturation in the low 90s (about 92%) which I believed was not uncommon for me. They found my blood pressure higher than normal (+30/+30) and attributed that to adrenal response to the stress. I requested oxygen but was not provided that; I believe the medical responders wanted to see my oxygen saturation first; and after they did; they felt there was no need for supplemental oxygen. I discussed symptoms with the EMTs and asked would they mind checking my cannula to see if it flowed oxygen. Isaid that I recalled that; prior to flight; when I turned it on; I did not feel the usual cool flow of gas; but I was satisfied as I double-checked the flow meter and found it responsive to adjustments. They also used a CO monitor to 'sniff' the aircraft cabin for presence of that gas; they found nothing remarkable. But; they did find no oxygen flow from the conserving cannula and brought it to me to show me two cuts nearly completely through the clear plastic line after the flow meter and before the cannula. Case solved ! The EMT's also conducted a 4 probe EKG and found those readings very normal. After 10-15 minutes in the ambulance; my oxygen saturation had reached 96-97%.My lessons learned are to inspect my critical equipment to include all oxygen equipment; to pay attention and investigate further when something is not as I expected (the lack of cool oxygen flow into my nose). I also learned that my body does not quickly recover from insufficient oxygen. I am glad I reacted immediately by descending safely but rapidly; and I was glad to have the autopilot and other automation to help me when impaired. Paying attention and taking quick corrective action was critical; so my additional lesson learned was to confirm the need for quick action. I am lucky to have had low terrain available from my position and a nearby airport. From when I commenced my descent; it took 11 minutes to get to traffic pattern altitude and 16 minutes to reach the runway surface. Even after landing; I felt too poorly to taxi. So it's good I landed when I did.