Copenhagen/Jutland/Amsterdam/Belgium

Yes there is an overnight ferry from Frederikshavn to Oslo.

Currently you still book it through DFDS, the old operators, although it is now run by a company called Gotlandsboget.
The route takes Foot Pax.
There is also a route from Hirsthals to Kristiansand, train to Oslo (on the Stavanger to Oslo line). It is run by Color Line with some sailings extending to Bergen via Stavanger.
That route also takes Foot pax.

It's half way back to Copenhagen but there are also Billund to Oslo flights with SAS.

Hello,

I will build on the answer provided above.

First, a small but important correction: The DFDS/Gotlandsbolaget ferry Frederikshavn - Oslo will no longer be stopping in Frederikshavn, from end of March 2025 until mid of December 2025 (where it seems like they will be stopping in Frederikshavn again due to the ski season). For context, the current full route is Copenhagen - Frederikshavn - Oslo, and in the future it will become a direct non-stop route Copenhagen - Oslo. This means Frederikshavn will (once again) lose its ferry connection to Oslo. It's a relatively new change, so understandable that a lot of people don't know this yet.

Other ferry routes that could be useful: There are also Color Line ferries Hirtshals - Larvik (then trains to Oslo). There are also Stena Line ferries Frederikshavn - Gothenburg (then trains to Oslo).

Yet another flight alternative: SAS also flies to Oslo from Aalborg (in northern Jutland).

Personally I would not combine Scandinavia with Netherlands/Belgium, even with 3 weeks. I suggest that you develop a proposed itinerary, and include (estimated travel time) between sleeping places. I am tired of airport security and late flights. I also don't care for bargain airlines that restrict my luggage choices, but I pack heavy.

Have you considered flying out of Brussels BRU so you don't have to return to Amsterdam? That adds a whole day to your vacation. Or if the air options suit, fly to BRU and end up in Amsterdam. I trust you know that there are many optional unreserved train daytrips (around 1 hours) from Amsterdam. Are you trying to Keukenhof by May 11, 2025?

What is the thought process behind this trip? It's a very odd combination of places. Any reason for spending a few days in Oslo only to continue Amsterdam?

Don't drive from Copenhagen to Odense, take the train. It will save you time and money. Then you can rent a car in Odense, or continue by train a bit and rent the car somewhere in Jutland.

Lots of things to ponder itinerary-wise and invaluable information on ferries/trains. It's no wonder I've been so frustrated while trying to find information.
We don't have experience doing trips with multiple countries and we would like to explore both of these areas so it seemed like a good idea to combine them. We are relaxed travelers with no need to rush about seeing everything. You've given me pause to reconstruct our trip and it's appreciated.

There is nothing wrong with doing a a trip that involves multiple countries, but it's usually good to take geography into account when planning. If you're flying to Copenhagen and home from Amsterdam I'd suggest that you focus on Denmark, Germany and the Netherlands.

Info: Two main events in your dates of travel:

• Hamburg port anniversary (Germany), May 9-11
• Constitution Day, e. g. Oslo in Norway; May 17

Ferry by Colorline available from Kiel (Germany) to Oslo.

Just an idea: Flying into Hamburg, renting a car there after celebrations (cheaper than in CPH or OSL) and doing a round trip back to Hamburg. From there train to Amsterdam and Belgium.

Well you've all encouraged us to change things up. Since our flights are booked we will keep Denmark for the initial portion and then fly to Netherlands for the last 2/3ish.
We will keep Scandinavia area for a separate trip.
As soon as I made that small change I was so much more relaxed and now looking forward to a lovely trip. Thanks for the great ideas and encouraging us to rethink the itinerary.