"__o" is not declared in Visual Studio 2008 doing ASP.NET

Answer 1

Just add this on the page at the top:

<%="" %>

...So now my ASP.Net content placeholder begins like this:

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<%="" %>

Quote from: Mikhail Arkhipov (MSFT)'s WebLog

In order to provide intellisense in <%= %> blocks at design time, ASP.NET generates assignment to a temporary __o variable and language (VB or C#) then provide the intellisense for the variable when you type in the <%= %> block, similarly to what happens when you type x=.

Answer 2

This was discussed in length on the ASP.NET Forums. You should be able to find an answer there on the linked thread.