The one2many fields store the values in a format described on the write method of the model. For this task you need to pass the one2many field for the on_change method and check the state of their records in order to properly sum your total, something like this:

 def onchange_line_ids(self, cr, uid, ids, order_lines, context=None):  
    total = 0
    line_pool = self.pool.get('sale.order.line')
    for line in order_lines:  
         if line[0] == 0:
             total += line[2].get('qty', 0)
         elif line[0] == 4:
             total += line_pool.browse(cr, uid, line[1], context=context).qty
         elif line[0] == 1:
             if line[2].get('qty', False):
                 total += line[2].get('qty', 0)
             else:
                 total += line_pool.browse(cr, uid, line[1], context=context).qty  
    return {'value': {'total': total}}