your confirm page should be storing the order information in the database using the eCart Store order Summary and Store Order details behaviors.
when the Store Order Summary Behaviors runs, it will create a session variable that contains the OrderID value from the primary key of the orders table.
Double click the store order summary behavior on the confirm page and make note of the session variables name.
You should be able to find the session variable listed in the session section of the bindings panel. This binding will give you the code to use to show the OrderID.