All our OM-1/1N repairs include conversion to the 357 silver oxide battery (MS76, KS76, SR44W, G-13, 303). Meter circuit mod includes a diode for stepping the voltage down to 1.35V. The 625A or 625U battery that is sold to unsuspecting buyers is not a real replacement for the mercury 1.35V cell. First, the 0.2V difference causes a 2-3 stop error in the OM-1 / 1N meter. Since it is alkaline, the voltage starts to fall as soon as you start to use it. Any calibration that is done with that battery is almost immediately lost. The zinc-air 1.35V battery (really 1.4V) will cause a 1/2 (one half) stop meter error and, once activated, that battery goes dead in a few months whether you use it or not . If the meter does not work with these batteries, it is most likely because the meter was already defective before the battery was replaced. If you do not want to invest in an overhaul to get your camera converted to the 1.55V silver oxide battery, you can buy an MR9 adapter, which drops into the battery box, and uses the 1.55V 386 silver oxide battery.
Sometimes I see OM-1 & 1N bodies with unusually stiff meter movement. Needle should casually swing up and down. For those that don't move smoothly, here's a close up of the possible meter defect. Note this is only found after the meter movement itself is disassembled (outer metal case removed from the meter movement.) This defect usually caused by impact damage.
Found in oldest style OM-1 bodies, the outer case of this meter has a small circuit riveted to it. Then 2 brushes rub on this board. Olympus Japanese techs think this could have been part of an undeveloped battery test circuit or it could be a switch that closes to warn the user if he / she is trying to center the needle with the meter turned off. The meter turns so far off the mark, that the brushes touch the wide part of the metal pad and the needle goes to the top. Could be! Though my Japanese trainers at Olympus referred to it as "battery check".

OM-1/1N meter needle WILL move with the switch off, with dead battery, no battery or even with a bad meter winding. It will usually center around ISO 100, f1.8 and shutter speed "B".
Tech Pages - OM-1/1N pt. 2
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OM-1/1N Best Battery: 1.35V mercury cell, LONG discontinued. My overhauls include conversion to the 1.55v Silver Oxide battery. Result is accurate and linear. If you're tempted to use 625A or 625U alkaline batteries instead, they cause 2-3 stop error.
Another defect now found in OM-1/1N meters:  balance spring (there are 2) can come unsoldered from coil in ceneter of meter.  No way to resolder this, replacement meter adds $ to the repair cost. 
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