$ opatch lsinventory
Invoking OPatch 10.2.0.3.0
Oracle Interim Patch Installer version 10.2.0.3.0
Copyright (c) 2010, Oracle Corporation. All rights reserved.
Oracle Home : /ora/app/product/10.2.0/db_1
Central Inventory : n/a
OPatch version : 10.2.0.3.0
OUI version : 10.2.0.3.0
OUI location : /ora/app/product/10.2.0/db_1/oui
Log file location : n/a
Patch history file: /oracle/product/10.2.0/db_1/cfgtoollogs/opatch/opatch_history.txt
OPatch cannot find a valid oraInst.loc file to locate Central Inventory.
OPatch failed with error code 104
oraInst.loc is there in Oracle Inventory directory. In my case, my oracle inventory is /oracle/oraInventory and oraInst was there in /oracle/oraInventory
copy oraInst.loc in /etc as oracle user and issue :