When you want to use SQL*Plus or exp/imp on the same server on which you have installed your database you might get errors like these:

sqlplus: error while loading shared libraries: libsqlplus.so: cannot open shared object file: No such file or directory

or

Error 6 initializing SQL*Plus
Message file sp1us.msb not found
SP2-0750: You may need to set ORACLE_HOME to your Oracle software directory

Syntax:
$ sqlplus username/password

You get this error only when you execute ‘sqlplus’ as a user outside of the dba or Oracle group, because in Oracle Database 10g Release 2 the Oracle Home was locked down by setting umask to 007 prior to installation, so that the files that sqlplus needs to execute do not have read/execute permissions for group “others” and some directories under $ORACLE_HOME have no world read nor execute permission (rwxr-x—)

Warning , don’t use the following command to lower the permissions, this may corrupt your Oracle-Installation:
chmod -R 755 $ORACLE_HOME/

The better way is to use an script which is provided by oracle:
$ORACLE_HOME/install/changePerm.sh

This script will relax the permissions on most files and directories in your Oracle Home, and allow “others” to execute them.