The following, taken from here, was not mentioned here:
If the Oracle Client is 32 bit, it will contain a "lib" folder; but if it is a 64 bit Oracle Client it will have both "lib" and "lib32" folders.
Also, starting in Oracle 11.2.0.1, the client version for 64-bit and the Oracle client for 32-bit are shipped separately, and there is an $ORACLE_HOME/lib64 directory.
$ORACLE_HOME/lib/ ==> 32 bit $ORACLE_HOME/lib64 ==> 64 bit
Or
$ORACLE_HOME/lib/ ==> 64 bit $ORACLE_HOME/lib32 ==> 32 bit