• User ID: Very obvious
• Password: Also obvious
• Data Source: MSDN says Server name

This makes a connection string like this:

User ID=MyName;Password=Asecret007;Data Source=server_database

But actually this is the name in the TNSNames.Ora file that maps to a full blown connection string.

The TNSNames.Ora can be found (on my system):

C:\oraNT\NETWORK\ADMIN\Tnsnames.ora.

You can built it using the Oracle Net Configuration Assistant. Choose "Local Net Service Name configuration" to Add, modify or delete.

I noticed that the OracleConnection fails when the Data Source is longer as 16 characters.

Example:

try {
    using (OracleConnection con = new OracleConnection(
             "User ID=MyName;Password=Asecret007;Data Source=server_database")) {
        con.Open();
    }
    MessageBox.Show(this, "Connection successfull", "Success");
} catch (Exception ex) {
    MessageBox.Show(this, "Connection failed\n" + ex.Message, "Connection failed");
}