Here is a DSN-less connection string.
Below: put your userid, password and oracle server name.
ex, UID= anagarur
PWD=annnap
SERVER = ntoracle.base.net
Leave the driver part as it is.

Cn = "UID=youruserid;PWD=yourpassword;driver=" _
& "{Microsoft ODBC for Oracle};SERVER=youroracleserver;"

Set Cn = New ADODB.Connection
With Cn
.ConnectionString = conn
.CursorLocation = adUseClient
.Open
End With

Hope this helps.