Such would be a strong indicator that something is continually generating gaseous oxygen. Otherwise, the O2 usually ends up oxidizing anything it can touch, and gets bound up in minerals, or as oxygen-containing gases.

Awesome for potential terraforming would be an atmosphere containing gaseous NO2, H2O, CO2, NH3, and SO2. That would be oxygen-rich, but not as molecular O2, which is almost as good as putting up an "OCCUPIED" sign on the planet.

Unless, of course, you're actually looking for alien life. In that case, any atmosphere with F2, Cl2, O3, O2, NO, CO, H2, or N2 is nearly a dead giveaway. Those don't persist for long in that form on a geological time scale, so if you detect them, something must be continually generating them.

It would, but the Earth Similarity Index doesn't consider the atmosphere.