The best replacement for oxygen concentrator is liquid oxygen.

The best alternatives to oxygen concentrator are rated as following:
★★★★★  liquid oxygen
★★★  oxygen generator
★  tank oxygen
★  oxygen cylinder
★  bottled oxygen

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Speaking about oxygen concentrator, one probably shouldn't forget about cpap.

Other search terms related to oxygen concentrator and their ratings are:
★★★★★ cpap
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oxygen concentrator
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oxygen generator