CS0234: The type or namespace name 'OracleClient' does not exist in the namespace 'System.Data'

i have created a class and defined a function in it. i have included using system.data.oracleclient in the .cs file i get this error, if i remove the using

system.data.oracleclient app works fine

 .

what is the solution to below problem,

Compilation Error

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.  Compiler Error Message: CS0234: The type or namespace name 'OracleClient' does not exist in the namespace 'System.Data' (are you missing an assembly reference?) Source Error:

Line 14: using System.Net.Mail;
Line 15: using System.Web.Security;
Line 16: using System.Data.OracleClient;
Line 17: using System.Data.SqlClient;
Line 18: 

Source File: c:\Users\ismail\Documents\Visual Studio 2010\Projects\WebApplication1\WebApplication1\App_Code\CommonMethods.cs    Line: 16