I was in this field  20  years ago and I'm trying to  recall 
some simple methods - not asking for a lot of money.
 http://www.madehow.com/Volume-4/Oxygen.html#:~:text=Oxygen%20can%20be%20produced%20from%20a%20number%20of,oxygen%20in%20the%20air%20back%20into%20carbon%20dioxide.

the picture below   is not closely related to  the link from  above
Wesley

We see here that indian scientists and inventors are in search for -cheap and available- oxygen generator/concentrator solutions since 2020 ( by medical grade quality ! ):


https://www.google.com/search?q=low+cost+oxygen+generator&ei=plGEYPmeLYaSlwTJmJmgBA&oq=low+cost+oxygen+generator&gs_lcp=Cgdnd3Mtd2l6EAwyBAgAEBM6BwgAEEcQsAM6BwgAELADEEM6BggAEAcQHjoICAAQCBAHEB46CAgAEAgQDRAeOggIABAHEB4QE1Ci3QRYzfIEYPmCBWgBcAJ4AIABtAGIAYgHkgEDOS4xmAEAoAEBqgEHZ3dzLXdpesgBCsABAQ&sclient=gws-wiz&ved=0ahUKEwj5iZq3sJfwAhUGyYUKHUlMBkQQ4dUDCA4


https://indianexpress.com/article/north-east-india/tripura/tripura-goldsmith-builds-low-cost-oxygen-concentrator-to-help-covid-patients-7073896/

 This was when her family learnt that an oxygen concentrator costs Rs 65,000. And renting an oxygen cylinder wasn’t cheap either; it was Rs 1,500 per day.

https://www.thebetterindia.com/223536/iisc-scientist-india-covid-19-coronavirus-pandemic-oxygen-generator-ventilator-medical-device/

“There are oxygen generators available in the market, but each unit costs anywhere from Rs 40,000 to Rs 1 Lakh. I realised that by using readily available materials, we can make it at a fraction of the cost,” Prof. Praveen Ramamurthy, one of the team members, tells The Better India (TBI). Their device costs less than Rs 10,000.


https://www.deccanherald.com/city/bengaluru-infrastructure/this-low-cost-oxygen-generator-could-help-save-covid-19-patients-in-rural-areas-828330.html


"This unit, which uses locally sourced items, can also supply 10 litres per minute. It will cost between Rs 10,000 to Rs 20,000. This includes all electronics, low-cost compressors and zeolite available in abundance. The plan is to transfer the open source design, free of cost, to manufacturers,” Dr Ramamurthy said.



125 US$- 250 US$/10 000 - 20 000 Rupies per unit   ,   lease/renting option ?!






But not a new global problem, soon before Corona-Pandemia :


https://www.unicef.org/innovation/oxygen-concentrators


Every year, 4.2 million children suffering from severe pneumonia in low and middle-income countries urgently need oxygen to survive.


Adults with pneumonia global,with "medical" oxygen need ?




#02 : this used "(Parker Whatman) Balson 75-31 Hydrogen generator " is with 2021 production condition as new model(l) and by same output never to get under 1500 US$ fob factory !


                                                  model(l)  75-31/ 75-32 ?
 http://www.triadscientific.com/en/products/nitrogen-generators-hydrogen-generators-zero-air/2673/parker-balston-whatman-hydrogen-generator-75-32/248648




Euphoria/Hysteria : https://translate.google.com/translate?sl=de&tl=en&u=https://www.startnext.com/diy-beatmungsgeraet

the idea : The aim is to develop a ventilator or an oxygen concentrator, to build corresponding prototypes and to share the designs as open source.


the great support !









Beside oxygen generator/concentrator also ventilator ,April 2021 development stage (me)  unknown :

https://translate.google.com/translate?sl=de&tl=en&u=https://healthcare-in-europe.com/de/news/forscher-entwickeln-beatmungsgeraete-zum-selbstbauen.html

https://translate.google.com/translate?sl=de&tl=en&u=https://www.heise.de/hintergrund/WirVsVirus-Beatmungsgeraet-im-Eigenbau-4690584.html


https://translate.google.com/translate?sl=de&tl=en&u=https://www.deutschlandfunkkultur.de/maker-szene-beatmungsgeraete-im-eigenbau.1264.de.html?dram:article_id%3D473529


professional legal replication model : https://www.oxicur.de/kur-5000 ION S



Oxygen concentrator
Dimensions: H 55 x W 21.5 x D 55 cm
Weight: approx. 21 kg
Operating voltage: 230 V / 50 Hz
Nominal current: 2.0 A
Flow: 0 - 5 l / min. stepless
Oxygen concentration:
0.1 - 3 l / min. 95 +/- 3 vol.% O2
- 4 l / min. 90 +/- 3 vol.% O2
- 5 l / min. 82 +/- 3 vol.% O2


Sound pressure level: 48 dB (A) according to ISO 3744
Outlet pressure: 300 - 350 mbar
according to DIN EN ISO 8359
Coarse filter: accessible from the front
Fine filter: 2-fold in the device
Microfilter: in the device
Bacteria filter: in the device


comparison : weight/liter per minute/%


https://www.aliexpress.com/item/1005001425171573.html?albagn=888888&&src=google&albch=search&acnt=479-062-3723&isdl=y&aff_short_key=UneMJZVf&albcp=12578137352&albag=122351024049&slnk=&trgt=dsa-52949609153&plac=&crea=507686942134&netw=g&device=c&mtctp=b&memo1=&albbt=Google_7_search&aff_platform=google&gclid=EAIaIQobChMIquvS89iX8AIVDrLVCh17XAV_EAAYBCAAEgLU_vD_BwE&gclsrc=aw.ds

5 lt/min

https://www.alibaba.com/product-detail/5lpm-home-oxygen-concentrator-mini-oxygen_62371708932.html?spm=a2700.7724857.0.0.55604c5aCcJVxX


250 US$ for a 10 lt/min 95% oxygen concentrator/generator is a good target !


https://www.youtube.com/watch?v=8fDJ30SG4NA     https://www.youtube.com/watch?v=2U1sdMwJ1gE


https://www.youtube.com/watch?v=kibjku2GYq0        https://www.youtube.com/watch?v=hJ9agj3yWJc

It looks as if electrolysis would work fine but is ruinously expensive. Stackexchange suggests 5,000 amps at 1.8v with 4kw of heat to be handled somehow and Quora proposes that 8 minutes of flow costs 1KWH.
Maybe oxygen concentrators could become a DIY project. In simplistic terms, it seems to be applying air at high pressure to a zeolite plug. The O2 goes through and the N2 is absorbed. Releasing the pressure allows the N2 to escape, and so the cycle repeats. (An oscillating IC engine like solution would be interesting).