$ cat /etc/odbc.ini
[MYSQL]
Trace    = Off
TraceFile= stderr
Driver   = MySQL
DSN      = MYSQL
SERVER   = localhost
USER     = root
PASSWORD =
PORT     =  3306 
#OPTIONS =  1 
DATABASE= test
#SOCKET  = /tmp/mysql.sock

[ORACLE]
Driver          = ORACLE
Database                = ORCL
User            = scott
Password                = tiger
METADATA_ID             =  0 
ENABLE_USER_CATALOG             =  1 
ENABLE_SYNONYMS         =  1 

#include <qapplication.h>
#include <qsqldatabase.h>

#define DB_SALES_DRIVER     "QODBC3"
#define DB_SALES_DBNAME     "test"
#define DB_SALES_USER       "root"
#define DB_SALES_PASSWD     ""
#define DB_SALES_HOST       "localhost"

int main( int argc, char *argv[] )
{
        QApplication app( argc, argv, FALSE );

        QSqlDatabase *defaultDB = QSqlDatabase::addDatabase( DB_SALES_DRIVER );
        defaultDB->setDatabaseName( DB_SALES_DBNAME );
        defaultDB->setUserName( DB_SALES_USER );
        defaultDB->setPassword( DB_SALES_PASSWD );
        defaultDB->setHostName( DB_SALES_HOST );

        if ( ! defaultDB->open() ) {
            qWarning( "Failed to open database: " + defaultDB->lastError().text() );
            return FALSE;
        }

        return TRUE;
}

g++ -o tst test.cpp \
 -I/usr/lib/qt-3.3/include \
 -L/usr/lib/qt-3.3/lib -L/usr/X11R6/lib -lqt-mt

$ ./tst
Failed to open database:  [unixODBC][Driver Manager]Data source name not found, and no default driver specified  QODBC3: Unable to connect

QODBCDriver::open: Warning - Driver doesn't support all needed functionality (1014). Please look at the Qt SQL Module Driver documentation for more information.