The US models cannot do not do blood oxygen due to the stoush with Masimo. They are different models I believe, solely for the US market marked LW/A - with sensors disabled. I have no idea if they can be reactivated by software update in the future if the lawsuit is settled.

It's interesting that the sleep apnoea monitoring doesn't use oxygen sats but only accelerometry. Possibly to get round the Masimo lawsuit. But if oxygen measurement is important for you then don't get a US Apple Watch! Presumably also an issue for Chinese sourced 15Pro and higher iPhones and Apple Intelligence.

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