Thanks for your help. We've found the answer now. We needed to put the decode statement into a HAVING clause:
SELECT order_id,
decode(status, 'Held', '1 - Held', 'Shipped', '2 - Shipped', '') as ordstatus
from order_header
having decode(status, 'Held', '1 - Held', 'Shipped', '2 - Shipped', '') = 'Shipped'