Where is Oxygen App ? Still missing in Apple Watch 10 ?

it never went away in models outside of usa, so if it's a feature in greece then it's still there in newer models,

you can always check such things by choosing your country at the bottom of the page so you get the local apple info stuff

Apple Watch - Σύγκριση μοντέλων - Apple (GR)

not if purchased in usa

The ability to measure blood oxygen is no longer available on Apple Watch units sold by Apple in the United States beginning January 18, 2024. 

from

Apple Watch - Compare Models - Apple

OP wrote " If I see correctly Oxygen app is still missing from newly launched Apple Watch 10. Is there a turnaround for this ? "

The iWatch Series 10 is available to Pre-Order

As of this writing, the iWatch 10 Series has yet to be released

watchOS - Feature Availability - Apple