Oxygen detection in space isn't hard, the article in question seems to be an attempt at detecting oxygen on an exoplanet which is massively hard.

Now, conversely, oxygen detection in galaxies/around stars is difficult for another reason. It doesn't want to remain free oxygen and will gladly bind with any other atoms/molecules it runs into.

https://now.northropgrumman.com/what-happens-to-oxygen-in-sp...