Relatively simpler devices (e.g. transport ventilators) ’sense’ oxygen input as the presence of a high pressure inlet gas – but does not measure actual oxygen concentration (FIO2). So long as the input pressure was adequate, many devices could not discern whether the concentration of oxygen input is adequate (i.e. 93-99%).
The more sophisticated and newer ICU ventilators and anesthesia machines have internal O2 analyzers. If these devices are connected to a gas input that is of adequate pressure but low oxygen concentration, they may either alarm or not function properly. The alarm may have to be disabled but this will vary by manufacturer.
In summary, if you connect a source of oxygen with low concentration, some ventilators may have no idea and not alarm, but other ventilators may sense this and not function properly or alarm (which you may or may not be able to disable).