There are two versions of the OmaPosti Pro API
Major difference, is that version 2 swaps the sender and receiver in the shipment data and labels, when doing a separate return shipment.
Version 2 is the recommended version, with a more secure authentication.
Version 1 documentation:
Please see this document for the description.
Version 2 documentation:
Please see the instructions in the new developer portal.
You can download OmaPosti Pro API examples from here.