Operations Manager Report subscription wizard asks you to specify a File Name when you choose Windows File Share delivery method. In most cases where you just need to have a most recent report delivered to the share you would just type a desired name in the field and look for the result then the time comes. If you would like to keep the history though, you would probably want to include something like a time stamp in the file name to keep the names unique and prevent reporting server overriding the results every time a new report is generated. To do the trick you can add @timestamp variable to the File name. For example: Report@timestamp. The resulted file name will look something like Report2009_10_05_000000 which would not only allow you to keep the names unique but would also give everyone an immediate idea on when the file was generated.
Read more about File Share Delivery in SQL Reporting Services documentation.