I ran into this little error with MVC where a bunch of errors showed up about  __o  not declared. This was driving me nuts. Then I ran across this link that solved it.

http://stackoverflow.com/questions/750902/how-do-i-get-rid-of-o-is-not-declared

So, the solution is to put this into the top of the page like VS does for your site.master.

<%-- The following line works around an ASP.NET compiler warning --%>
<%: ""%>

But what about other pages?

Lets say you have a view that’s using your site master and that view is throwing this error. Just add the items into the content section where the error occurs like so:

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

<%-- The following line works around an ASP.NET compiler warning --%>
<%: ""%>

Then add the rest of your code. That seems to fix it and its pretty simple too.