Order IDs are unique to each purchase. You can view it in the Order Confirmation email that you received for each purchase or booking, or alternatively in your purchase history at the merchant's account.
If you have lost your Order Confirmation, you can request for one from the merchant.
You can view the samples for Order IDs below:
|Store / Merchant Name||Order ID Sample||
Sample of Order Confirmation Proof
(Right click to enlarge)
|Hotels.com||1518808XXXXX OR 81181343XXXXX|