Unir dos consultas sql en funcionamiento obtiene resultados incorrectos:¿dónde me equivoco?

En tu join condición, utilizó orders_id . Debe usar orders_products_id así:

SELECT op.orders_id, op.products_id, op.products_model, op. products_name,
       op.final_price, op.products_quantity, opa.products_options_values_id,
       opa.products_prid
FROM orders_products op
JOIN orders_products_attributes opa ON op.orders_products_id = opa.orders_products_id
WHERE op.orders_id = 21535