Most ventilators operate with 35-65 psig/4 bar inlet which is sufficient to operate the blender (i.e. what mixes 100% oxygen and air). Without adequate pressure the device may not function at all or with inaccurate delivery of volume, pressure and oxygen concentration. If a reliable high pressure gas supply is not present, consideration of ventilators with turbines or compressors may be critical.
Some ventilators that measure FiO2 will alarm if you connect something less than 100% oxygen to them.  Some ventilators (especially transport ventilators) may not have an FiO2 sensor and just display the blender fraction setting, not a direct measurement of concentration (this requires calibration during preventative maintenance).