it's not rare in combination with other elements. it is rare on its own, as it is so reactive. there would be very little free molecular oxygen in earth's atmos

it's not rare in combination with other elements. it is rare on its own, as it is so reactive. there would be very little free molecular oxygen in earth's atmosphere if it was not being actively produced via photosynthesis and any life we encounter is unlikely to be based on anything but carbon, because it is the only element capable of forming likely complex structures because of thermodynamics, among other things