Search notes:

$ORACLE_HOME/OPatch/opatch

opatch is typically used to patch files located under the Oracle Home (such as binaries or timezone files) and needs therefore to be run while the instance is down.
This is in contrast to datapatch which patches data «inside» the database.
$ORACLE_HOME/OPatch/opatch is a shell or .bat script. 
Usage: opatch [ -help ] [ -report ] [ command ]

Available commands

apply Applies an interim patch to an Oracle Home. System patches are not supported with this command. Compare with napply.
compare Compares two files produced with opatch lsinv -xml
lsinventory Lists the inventory for a particular $ORACLE_HOME or display all installations that can be found.
lspatches Lists and verifies patch metadata specified by patch ID (installed in Oracle Home) or patch location.
napply Applies a set of patches at a time. Like apply, system patches are not supported with this command.
nrollback Rolls back a set of patches at a time
rollback Roll back an existing patch indicated by a reference id.
query Loads and prints out various information about a patch.
version Prints the version of OPatch for a particular Oracle home or displays all OPatch versions in the central inventory.
prereq Runs the prerequisite checks on an Oracle Home.
util invokes the specified utility in an Oracle Home

lsinventory

The current Patch Set Updates (PSU) and interim patches can be determined like so 
$ $ORACLE_HOME/OPatch/opatch lsinventory
$ $ORACLE_HOME/OPatch/opatch lsinventory | grep 'Patch description'
Compare with the following SQL statement: 
select * from sys.registry$history;
The information that is available with opatch lsinventory -xml can also be queried using the dbms_qopatch package.

See also

The $ORACLE_HOME/OPatch directory.
Contents of a patch file
Each executed OPatch command is recorded in $ORACLE_HOME/cfgtoollogs/opatch/opatch_history.txt.
dbms_qopatch
Patching Oracle

Index