Hi,
We'd need more information about any errors in your logs in order to help. OAI-PMH harvesting should work properly in DSpace 7.6. if you are receiving errors, then I"d recommend using our Troubleshooting guide to locate the detailed error message in either the User Interface or backend logs.
https://wiki.lyrasis.org/display/DSPACE/Troubleshoot+an+error#Troubleshootanerror-DSpace7.x(orabove) Without that detailed error message, it's very difficult for anyone to help. Sometimes errors can be caused by configuration issues, or might be unique to your site based on the external service you are harvesting *from*.
Tim