Humans in the 3300's seem to require very little Oxygen, since the lowest ratio of Oxygen on an ELW can be as low as 2%.
The actual percentage of oxygen is irrelevant for habitability calculations. What's important is the partial pressure of oxygen. Example: a planet with only 0.5 atmospheres, but with 42% oxygen (twice the Earth percentage of oxygen) in the atmosphere, would have exactly the same oxygen levels as Earth does: a partial pressure of 0.21 atmospheres. The air on such a planet would "feel thin", and Earth-birds and Earth-insects would be unable to fly due to the thinner air, but it would be perfectly breathable indefinitely. Likewise, a planet with twice the total pressure but half the oxygen percentage would have the same partial pressure of oxygen to Earth.
That Earth-like planet in Falconfly's post above, with 0.55 atmospheres and 28.7% oxygen, is dangerously low-oxygen - it has only 0.157 atmospheres partial pressure of oxygen, or the equivalent of 15.7% Earth-normal-pressure oxygen. Humans can survive with oxygen levels that low, so long as they're not physically exerting themselves too much, but it's a very low quality of life. Below 0.14 atmospheres partial pressure of oxygen, even a completely motionless human won't get enough oxygen to prevent mental impairment - they'd be manually concentrating on deep, non-reflexive breathing so much that they wouldn't be able to think about anything else.
I think the cutoff in ED is a partial pressure somewhere around 0.15 atmospheres. That would mean the lowest in-game reported oxygen content of a procedurally-generated ELW should be around 3.6% for a planet with about 4 atmospheres total pressure (the highest total pressure a human can tolerate before the inert gases in the atmosphere become toxic). Such a planet would be super-uncomfortable for an unaugmented human to live on: pea-soup-thick air that made you work hard for every molecule of oxygen you got into your lungs. But even humans could take off and fly, just by strapping some wings onto their arms and flapping.
You might find "ELWs" in the bubble with total pressures way above 4 atmospheres (I think the record is somewhere around 8 or 9 atmospheres) and oxygen levels down around 2% (I believe I have seen such worlds), but those are hand-crafted anomalies that should not in practice exist. The Stellar Forge knows that such planets don't really have "Earth-like" conditions and when it makes such worlds, it doesn't call them "Earth-like".