I guess you have forgotten to include the providerName for your connection string.
If you are using ODT it should look something like this
<add name="ConnectionString" connectionstring="Data Source=xyz;User Id=SCOTT;Password=tiger;" providername="Oracle.DataAccess.Client" />
Or of you are using the ms oracle provider is should look like this:
<add name="ConnectionString" connectionstring="Data Source=xyz;User Id=SCOTT;Password=tiger;">
providerName="System.Data.OracleClient" />
</add>
Best regards
Espen Harlinn