There is a company in UK called 'The Small Battery Company' (Google the name) They sell a tiny device that fits into the battery compartment of cameras that used the defunct Mercury type. This device is then fitted with a suitable 1.5 silver cell but the voltage is reduced to 1.35 volts, the same as the mercury ones. There are alternatives such as the Wein Cell which also produce 1.35V but they are not cheap and do not last very long, or you could have the meter re-calibrated to handle 1.5V instead of 1.35V, but this could also be be expensive
I would concur with BMbikerider here. I have tried both solutions and the Small Battery Company adaptor works well and is worth the expense if you use the camera frequently. If you use the camera in bursts, shooting a few rolls of film then nothing for a long time or just want to put a few films through then sell the camera on, the Wein cells could work out cheaper. Bear in mind that if you do decide to sell the Olympus on, the adaptor can be retained and used with other cameras you may acquire which require the PX625 mercury cell should you buy one or even offered with the camera allowing you to charge a higher price and recoup some of the cost.
" ... a cook who relies on nothing but a sharp knife has no guarantee of producing excellent dishes." - Yoshihisa Maitani