The ORACLE_HOME environment variable must define the Oracle software
installation path.
The ORACLE_SID environment variable can be used to define the
name of the local database instance.
The PATH environment variable must define the access path to database
client programs.
On UNIX™, LD_LIBRARY_PATH
(or equivalent) must hold the path to $ORACLE_HOME/lib.
The Oracle client library 'ORACLE_HOME/lib/libclntsh*' must
be available.
The TNSNAMES.ORA file must define the database server identifiers
for remote connections (the Oracle Listener must be started
on the database server to allow remote connections).
The SQLNET.ORA file must define network settings for remote connections.
Make sure the database client locale is properly defined.
You can make a connection test with the Oracle sqlplus tool.