OIOUBL is the Danish default e-invoice format.
It support the following document types:
The official example files can be downloaded by clicking here: Download example files
The documentation can be found here: https://www.oioubl.info/classes/en/index.html
If a Danish recipient is receiving e-invoices, they will generally be registered in the network NemHandel.
They can be looked up via the site here: NemHandel lookup
They can also be registered for both PeppolBis and OIOUBL format in Peppol.
That can be looked up here: Peppol lookup