I'm confused about the scope of the lambda variable, take for instance the following

var query = 
    from customer in clist
    from order in olist
    .Where(o => o.CustomerID == customer.CustomerID && o.OrderDate ==  // line 1
        olist.Where(o1 => o1.CustomerID == customer.CustomerID)        // line 2
             .Max(o1 => o1.OrderDate)                                  // line 3
    )
    select new {
        customer.CustomerID,
        customer.Name,
        customer.Address,
        order.Product,
        order.OrderDate
    };

In line 1 I have declare a lambda variable 'o' which means I cannot declare it again in line 2 (or at least the compiler complains if I try to) But it doesn't complain about line 3 even though 'o1' already exists??

What is the scope of a lambda variable?

The brackets give the clue - the lambda variable is captured in the scope of where it's declared:

.Where(o => ... olist.Where(o1 => ...).Max(o1 => ...))
  //  |----------------------------------------------| scope of o
  //                       |---------|                 scope of first o1
  //                                      |---------|  scope of second o1

Note that there's no overlap for the two o1 variables, but they both overlap (or shadow) the o variable and hence can't use the same name.