!!
critical Omron
Error Code O
Batteries are depleted or monitor turned off unexpectedly during measurement.
? What Causes This
Batteries have no remaining charge.
✓ How to Fix It
Immediately replace all 4 batteries with new ones. Refer to sub-section 2.1.
Source: page 39 of the official manual
From the manual
This error code appears in:
Omron BP7250 Blood Pressure Monitor