You can either use the OleDb data provider in the System.Data.OleDb namespace, or download an oracle-specific provider. AFAIK, there are a number of such providers, such as the one from MS (http://msdn.microsoft.com/downloads/default.asp?url=/downloads/sample.asp?url=/msdn-files/027/001/940/msdncompositedoc.xml), from CoreLab (http://crlab.com/oranet/) and from Oracle itself (http://otn.oracle.com/tech/windows/odpnet/content.html).
From there on, you use the regular ADO.NET interfaces, which are homogeneous among all providers, such as IDbConnection, IDataReader, etc. A good overview of ADO.NET and its interfaces can be found in the .NET Show about it at http://msdn.microsoft.com/downloads/sample.asp?url=/MSDN-FILES/027/001/730/msdncompositedoc.xml
This was first published in February 2003