Update: Here are a list of things I think I'll try based on comments: 1. oxygen cylinder, 2. CO2 scrubber, 3. oxygen concentrator, 4. air exchanger.
Just like people who live at high altitudes adapt to the lower oxygen levels, except in reverse.
I wouldn't be willing to tolerate even a very small amount of toxicity just for increased productivity if that is the case.
I was always sceptical of how useful CO2 monitors are, but I was barely functional in that office. A colleague of mine regularly worked in the corridor because he got such bad headaches while in the office. For me I just got drowsy and couldn't focus.
I've never tried to actively increase oxygen, but I do carry a CO2 monitor now and I think if you're going to be doing mentally demanding tasks in a confided space it's useful to think about.
Personally I wouldn't want to increase O2 concentrations beyond levels you'd generally find outdoors though... Perhaps if you occasionally want a short burst of focus it wouldn't be too harmful, but I'm guessing its effects will be a bit like caffeine in that if you consume it constantly your body will just adjust to a new baseline and you'll find it hard to cope without it.
A suggestion from someone who has built three different CO2 monitors - don't chase the numbers. 420/800/1200 ppm has no meaningful difference, and if you don't have a good way to calibrate your CO2 sensor, such as a pure nitrogen environment or taking your sensor outside at least weekly and forcing a calibration, you can easily be 500 ppm out of true.
I suspect that you probably feel more clear-headed outside because there is wind and nature and distractions that aren't an LED screen.
That said, I think an interesting experiment would be to use a CO2 scrubber to remove carbon dioxide, instead of trying to overpower it with more oxygen. Otherwise, just get a tank of oxygen and face mask and see how it works for you.
1. Check your blood oxygen saturation with several pulse oximeters. They are not reliable enough to only use one.
1.1 You can buy sleep study watches that monitor your oxygen levels constantly and record / graph it, but they involve things that clip on your finger tip. It is hard to work with one on.
1.2 Most oxygen monitoring activity trackers are fake. Even brand names do not read the same as medical pulse oximeters and sleep study watches.
2. Open a window. If outside air is better, breathe outside air.
3. Monitor your air. You seem to have a CO2 meter. Get a second one and check if it is reliable. If they disagree, buy a third unit as a tie breaker. Compare your inside air to outside air, which should be below 400 ppm CO2.
3.1 Buy several oxygen meters and check the oxygen in your outside air (should be 20 percent, plus or minus 1 percent.) Check the inside air Oxygen meters can usually be calibrated.
4. If you use an oxygen cylinder, be aware of what can happen if you drop it. A mount that is screwed to a wall or a good stand is not optional.
4.1 if you use an oxygen cylinder, make sure it is medical oxygen and not contaminated with welding gasses.
4.2 if you use an oxygen cylinder, make sure you calculate the cost with a regulator, tubing, cannula, and frequency refills. They do not last long.
4.3 If you use an oxygen cylinder, make sure not to get too much oxygen in the room. Remember the Apollo 1 fire.
4.4 If you buy or rent oxygen cylinders, there are different sizes.
5. If you consider an oxygen concentrator, they are cheaper used, but bring your own tester and check them out. Many used ones are broken. Selling a broken oxygen concentrator can kill heart failure patients, but it happens.
5.1 If you buy an oxygen concentrator, it separates oxygen from surrounding air. That gives oxygen through a tube and oxygen depleted air around the concentrator. Keep a window open and a fan on for at least an hour a day.
5.2 If you buy an oxygen concentrator, do not put it in a box or closet to muffle the noise. It sucks the oxygen out of that area and then can not get any more oxygen.
5.3 If you buy an oxygen concentrator you will need tubing and cannulas.
5.4 I can imagine someone plugging in an oxygen concentrator without a cannula. It would separate oxygen from room air and dump them out 2 exit points, allowing them to recombine. This would be useless.
5.5 Oxygen concentrators heat up the room. Be prepared for as much heat as an extra PC workstation produces.
6. Oxygen toxicity is real, but I have no experience with it.
7. There are legends of guys putting several oxygen concentrators in a well ventilated out building / storage building and flooding a room in the main building with oxygen. This takes us back to the Apollo 1 fire and to oxygen toxicity.
greens brings comfort to the eyes, tending to them eases you out too. walking around or exercising gives you better oxygen circulation too.