Oxygen is a highly reactive element that on a cosmological time frame instantly disappears by bonding with things. It is very difficult to come up with an explanation other than "life" for why an atmosphere would be full of oxygen; it may well be impossible to come up with an explanation other than "life" for why an atmosphere would be full of oxygen for 500 million years, especially on a ball of iron. Even having a transient explanation would be very difficult.

Not all living organisms use oxygen (even on our planet), true.

But you don't get free oxygen on a planet without life to continually produce it.

It's a good indicator of some forms of life.

We don’t have a lot of examples of technological life forms that don’t need oxygen.