I have had problems in exporting a suitable list of completed peer reviews for the year, as a base to thank them at the end of the year. I could not find a workable report in OJS, but an excellent solution was the reports available through the ReviewerCredits plugin. If a journal is registered with RC -even with a free plan- it is possible to export nice and tidy lists which include statistical information for the Reviewer Contribution Index. I would recommend that anyone who faces my same problems has a go installing the plugin and registering their journals.
If you want to know more about how I used the reports and the RC options, just contact me.