Life on earth existed for ~1.2 Billion Years before the Great Oxidation Event seeded Earth's oceans and atmosphere with O2.
That is 1/3 of the entire time life has existed on earth.
Yes, finding a very reactive element like oxygen in the atmosphere is a great indicator of an ongoing organic process on an Exoplanet.
But in our sample size of one we already know that No free oxygen does not mean, no life. And while our sample size of one indicates that intelligent life comes after o2 in the atmosphere we have no idea if this is always the case.
Finding atmospheric o2 would be a great leap in exoplanet and exobiology research as something as reactive as free o2 is a massive pointer to an organic origin. Any free o2 would have to be constantly replenished as it would quickly be reacted with (quickly in geological timescales) and pulled from the atmosphere and there is no known normal planetary process to do this.
Mars is the perfect example as it is red due to all the iron-oxide (rust) in the surface regolith and there is basically none (less than 0.0001%) in the atmosphere.
The flipside of this is that without atmospheric oxygen, iron would not be locked up in iron-oxide and may be more available to use by non-oxygen breathing life forms. All the iron ore humans mine and use comes from the release of o2 into the world's oceans during the the Great Oxidation Event, which combined with dissolved iron in the oceans then sank to the bottom to become the deposits we now mine.