What is a PSU patch and How to verify the PSU update in the Oracle Home and database

What is a PSU patch for Oracle Home?Patch Set Updates (PSU) are the same cumulative patches that
include both the security fixes and priority fixes.  The key with PSUs is they
are minor version upgrades (e.g., 11.2.0.1.1 to 11.2.0.1.2).  Once a PSU is
applied, only PSUs can be applied in future quarters until the database is
upgraded to a new base version.

More details about PSU
The PSU   released for Oracle Database Server version 10.2.0.4.0/10.2.0.5.x , 11.1..0.7.0 and 11.2.0.1.0/11.2.0.2.0  respectively does not change the database version . That means PSU  10.2.0.4.x /10.2.0.5.x, 11.1.0.7.x, 11.2.0.1.x /11.2.0.2.x ( where x is the fifth digit ) does NOT change the 5th digit of the Oracle Database Server version.

opatch lsinventory” still shows the version as 10.2.0.4.0/10.2.0.5.0 for Oracle 10g R2, 11.1.0.7 for Oracle 11g R1  and 11.2.0.1 /11.2.0.2 for Oracle 11g R2

PSU’s also will not change the version of oracle binaries (like sqlplus, exp/imp etc.)

 How to Verify if PSU is installed in the ORACLE_HOME

$ opatch lsinv -bugs_fixed | grep PSU

85733156 88334280 Tue Feb 09 16:54:18 MST 2012 DATABASE PSU 10.2.0.4.3 (INCLUDES CPUJAPR2011)
8833280 883280 Tue Feb 09 16:54:18 MST 2011 DATABASE PSU 10.2.0.4.4 (INCLUDES CPUOCT2010)

Verify the PSU Post Install steps were run in the DB
select substr(action_time,1,30) action_time,
substr(id,1,10) id,
substr(action,1,10) action,
substr(version,1,8) version,
substr(BUNDLE_SERIES,1,6) bundle,
substr(comments,1,20) comments
from registry$history;