Solution : https://service.sap.com/sap/support/notes/719928 (SAP Service marketplace login required)
Key words : 
/oracle/con/920_64/opatch/opatch, terms opatch inventory reason, opatch rollback -id <patch_id> -, interim inventory update exception, update inventory error2, run 'opatch lsinventory', opatch versions lower, sap service marketplace, oracle universaler installer, uninstalled interim patches
Related Notes : 
       
| 640910 | Problems when using OPatch | 
| 629588 | Current versions of OPatch and OUI | 
| 539921 | Current patch set for Oracle 9.2.0 |