O2 has a service locator on their web site and I input Japan and it returned a positive for coverage. The 8310 is GSM so maybe a call to the carrier would help.
Almost everywhere in the work uses GSM networks and O2 has agreements with well over 424 of them in nearly 200 countries! Are you going somewhere special? Have a look at our guide to see what service you will be able to use - International coverage - Travelling abroad - International - O2