I have never own an OM-1 so I can't say much. But I have many cameras with similar meter design. Even with the correct power (that is you supply it with a rock solid external power supply at 1.35V) I have not seen one that is even accurate to 1/3 stop. It can be adjusted to be right on at 1 brightness level but not over a range say from EV5 to EV14 for example. That is why I don't really care for the meter on these type of cameras. I can simply estimate exposure.