Help with references in web project (Web Form Designer failed to load)

I added a reference to my web project but, when I try to view my page
design, I get the error:

"The file failed to load in the Web Form Designer".
The designer could not be shown for this file because none of the classes
within it can be designed".
"WebForm1 --- The Base Class 'System.Web.Ui. Page' cannot be designed".
This is everything I've included:

using System;
using System.Collecti ons;
using System.Componen tModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.Sess ionState;
using System.Web.UI;
using System.Web.UI.W ebControls;
using System.Web.UI.H tmlControls;//using System.Data.Ora cleClient;
using System.Data.Odb c;

I tried to delete the last reference I added (OracleClient) but I can't find
it.
Thanks,
VM