Their schedule is now open for the year. The Oslo-Copenhagen trip now depart at 15:00 and arrives at 10:00, a slight change from Ricks ebook. When should we be at the ferry for depart?
This page is a bit difficult to find on the DFDS website- https://www.dfds.com/en/passenger-ferries/passenger-information/terminals/oslo
Check-in opens: 2 hours and 30 minutes before departure*; Boarding starts: 2 hours before departure; Check-in closes: 15 minutes before departure; *Car check-in. For foot passengers, it is possible to check-in from 09.00
Can I get any discount on the ferry with a Rail Pass? or as a Senior? Thank you
No seniors discounts.
While Eurail does give discounts on a number of ferry routes, DFDS is not one of them. So it is full price only.
Two tips:
Most Baltic Sea ferries have discounts for early booking, especially if you travel by car.
You can also try to use the national pages for booking to get a better price. I do not know if it works for DFDS but for Fjord Line I saved around 30% for a car passage Hirtshals to Kristiansand by booking via the Norwegian website, not the German one.