This Olympus OM-1 uses a PX625 1.35V battery
Many older cameras and light meters were powered by the PX625 & PX13 1.35V mercury cell battery. The benefit of the mercury cell was that there was very little voltage drop over the lifetime of the battery. Unfortunately, most of these ended up in landfills, with the mecury eventually leeching into the groundwater. Because of this, mercury cell battery manufacturing has been banned in this, and many other countries. Alkaline 625 batteries are readily available but cannot be used because many vintage cameras used the constant 1.35V output of the mercury cell as a reference voltage and for powering the meter and any additional electronics. This could mean as much as 4 stops off on camera meter readings resulting in incorrect exposure. This wouldn't be too much of an issue if an alkaline battery supplied a constant voltage (after some experimentation, you would know how many stops to compensate), but it doesn't. A fresh 625 alkaline cell has a voltage of 1..5V, rapidly falls to 1.45V, and then falls slowly down to 0.9V.
1.4V 675 Zinc/Air with O-ring
Many substitutes are now available for the PX 625 such as the WeinCell MRB625 zinc/air battery. The drawback for these batteries is their relatively high cost and short lifespan. An zinc/air battery is activated by removing the paper cover allowing air to pass through holes into the battery where a chemical reaction occurs producing voltage. Once a battery has been activated, it is only a matter of months before it is exhausted. There are also many adapters (which regulate voltage by means of a schottky diode) on the market that allow the use of 1.5 alkaline and silver/oxide batteries but these are also relatively expensive.
Battery with O-ring Installed
This solution is cheap and all parts are readily available. All you need is an o-ring from the local home improvement store and a 675 1.4V zinc/air hearing aid battery from the drugstore. The variety pack of o-rings cost just under $3.00, and when purchased in multi-packs, the batteries can cost under a dollar apiece. Yes, being zinc/air, their activated lifespan is only a few months but they're cheap, and you have eleven more in the pack! O-ring measures approximately 14mm in diameter, and 2mm in section.
Bit of an electronics wiz? Find out how to make you own battery adapter HERE.
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