If you have installed Oracle 11gR2, the OPatch version will be 11.1.0.6.6 by default. If you proceed to apply any latest patches of 11.2.0.1, you must upgrade the OPatch version as well. The following error will be displayed if you try to apply a higher update of the patch and if your OPatch version is lower:

OPatch version    : 11.1.0.6.6
.......
ApplySession failed: Patch ID is null.
System intact, OPatch will not attempt to restore the system
OPatch failed with error code 73

From the logfile /u01/home/oracle/product/11.2.0/db_1/cfgtoollogs/opatch/opatch2012-12-15_11-57-48AM.log, the following message will be shown:

INFO:Starting ApplySession at Sat Dec 15 11:57:49 IST 2012
INFO:Starting Apply Session at Sat Dec 15 11:57:49 IST 2012
SEVERE:OUI-67073:ApplySession failed: Patch ID is null.
INFO:System intact, OPatch will not attempt to restore the system
INFO:Finishing ApplySession at Sat Dec 15 11:57:51 IST 2012

To fix this issue, first upgrade the OPatch utility by downloading Patch 6880880 for your OS and Oracle release. Of course, the patch number will be the same even though your database version and release will be different, but you just need to select the related OS and database version. The following screenshot illustrates the same:

Then, after downloading the patch and unzipping it in the $ORACLE_HOME location, you should be able to apply the patch as follows:

[oracle@oracle-stby patches]$ opatch -help
Oracle Interim Patch Installer version 11.2.0.3.0
Copyright (c) 2012, Oracle Corporation.  All rights reserved.