Option 01:

$active_group = 'default';
$query_builder = TRUE;

$db['default'] = array(
    'dsn'   => '',
    'hostname' => '192.168.0.109:1521/orcl', //hostname:db_port/service_name
    //'hostname' => 'localhost:1521/orcl',
    'username' => 'db_username', 
    'password' => 'db_password', 
    'database' => 'db_name',
    'dbdriver' => 'oci8',
    'dbprefix' => '',
    'pconnect' => FALSE,
    'db_debug' => (ENVIRONMENT !== 'production'),
    'cache_on' => FALSE,
    'cachedir' => '',
    'char_set' => 'utf8',
    'dbcollat' => 'utf8_general_ci',
    'swap_pre' => '',
    'encrypt' => FALSE,
    'compress' => FALSE,
    'stricton' => FALSE,
    'failover' => array(),
    'save_queries' => TRUE
);


Option 02:

$active_group = 'default';
$active_record = TRUE;
$db['default']['hostname'] = '(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.0.246)(PORT=1521))(CONNECT_DATA=(SID=orcl)))';
$db['default']['username'] = 'db_username';
$db['default']['password'] = 'db_password';
$db['default']['database'] = 'db_name';
$db['default']['dbdriver'] = 'oci8';
$db['default']['dbprefix'] = '';
$db['default']['pconnect'] = TRUE;
$db['default']['db_debug'] = TRUE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = '';
$db['default']['char_set'] = 'utf8';
$db['default']['dbcollat'] = 'utf8_general_ci';
$db['default']['swap_pre'] = '';
$db['default']['autoinit'] = TRUE;
$db['default']['stricton'] = FALSE;