But on Earth this process is slower than the rate of absorption of oxygen by the crust, so without photosynthesis the oxygen produced by photodissociation would be minimal.
Waterworlds, and icy worlds like Europa, could quite likely have oxygen atmospheres. In fact O2 is present on many of the icy moons of the Solar system.
According to A. Leger a better biomarker would be ozone.
There may be abiotic mechanisms to produce and retain O3 , but its detection in an exoplanetary atmosphere is likely to be an encouraging sign.The only way to have a significant amount of O3 in the atmosphere spectrum is that O2 is produced at low altitude, eg by biological photosynthesis...