blood oxygen app for apple watch ultimate 1

No. Apple cannot allow the tech to be used on US watches, whether with Apple's app or someone else's.

M1 Mac mini, macOS 14.5;  Watch, series 7 GPS/cellular, watchOS 10.5; iPhone 12 Pro, iOS 17.5; TV 4th Gen, tvOS 17.5; HomePods, audioOS 17.5 and Airport Express w/ Airplay 2

The patient violation ruling by a federal judge was in relationship to the hardware used, not the software. Thus hardware on models sold in US is disabled. The ruling only applies to Apple Watches sold in US.

I believe that a watch bought outside the US with functioning Blood Oxygen and then brought to the US, the function will be disabled with the first US update.