On AIX, login as the oracle user and change directory to the ORACLE_HOME/OPatch (Note: OP are in capital) directory:

cd $ORACLE_HOME/OPatch

Then issue the opatch lsinventory command. I use these options:

• all (will check all oracle installations in ORACLE_HOME)
• invPtrLoc (provides the file location of oraInst.loc, which will provide the oracle inventory location)

bash-3.00$ opatch lsinventory -all -invPtrLoc ../oraInst.loc
Invoking OPatch 10.2.0.4.3

Oracle Interim Patch Installer version 10.2.0.4.3
Copyright (c) 2007, Oracle Corporation.  All rights reserved.


Oracle Home       : /opt/oracle/product/10.2
Central Inventory : /opt/oracle/oraInventory
   from           : ../oraInst.loc
OPatch version    : 10.2.0.4.3
OUI version       : 10.2.0.4.0
OUI location      : /opt/oracle/product/10.2/oui
Log file location : /opt/oracle/product/10.2/cfgtoollogs/opatch/opatch2010-09-16_11-41-52AM.log

Lsinventory Output file location : /opt/oracle/product/10.2/cfgtoollogs/opatch/lsinv/lsinventory2010-09-16_11-41-52AM.txt

--------------------------------------------------------------------------------
List of Oracle Homes:
  Name          Location
   OUIHome1         /opt/oracle/product/10.2

Installed Top-level Products (2):

Oracle Database 10g                                                  10.2.0.1.0
Oracle Database 10g Release 2 Patch Set 3                            10.2.0.4.0
There are 2 products installed in this Oracle Home.


There are no Interim patches installed in this Oracle Home.


--------------------------------------------------------------------------------

OPatch succeeded.