Oslo - Kiel is operated by Color Line, who don't have an Interrail discount.

There are also other ferries to Norway, e.g.:

• Eemshaven - Kristiansand (HNL, no Interrail discount)
• Hirtshals - Kristiansand (Color Line, no Interrail discount)
• Hirtshals - Kristiansand (Fjord Line, with Interrail discount), but they are not so close to the station in Hirtshals.
• Copenhagen - Oslo (DFDS, no Interrail discount)
• Frederikshavn – Oslo  (DFDS, no Interrail discount)

(There are the main ones I think, more are listed here).

I recommend you to make your Eurostar reservation well in advance. 

There is a limited number of pass holder seats on the Eurostar between London and mainland Europe. On popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

If you don't get any result, press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653