I have a mysql query which is as follows 

```
(SELECT order_product.op_id,
        order_product.ocat_id,
        order_product.op_partnunber,
        order_product.op_name,
        order_product.op_upc,
        order_product.op_desc,
        order_stockavailable.osa_id,
        order_stockavailable.of_id,
        order_stockavailable.osa_stocka,
        order_category.ocat_name
FROM 
    order_product 
    LEFT JOIN order_category 
    ON order_product.ocat_id = order_category.ocat_id
    LEFT JOIN order_stockavailable 
    ON  order_product.op_id = order_stockavailable.op_id)
UNION 
(SELECT order_product.op_id,
        order_product.ocat_id,
        order_product.op_partnunber,
        order_product.op_name,
        order_product.op_upc,
        order_product.op_desc,
        order_stockavailable_attributes.id,
        order_stockavailable_attributes.of_id,
        order_stockavailable_attributes.opap_stock,
        order_category.ocat_name
FROM order_product 
    LEFT JOIN order_category 
    ON order_product.ocat_id = order_category.ocat_id
    LEFT JOIN order_stockavailable 
    ON  order_product.op_id = order_stockavailable.op_id 
    LEFT JOIN order_stockavailable_attributes 
    ON  order_product.op_id = order_stockavailable_attributes.op_id)
ORDER BY order_product.op_name

```

The query is givng error, T

`Table 'order_product' from one of the SELECTs cannot be used in global ORDER clause` 

I checked the MYSQL manual, but am not getting any clue, any help would be really great.