@Cookie-On-Tour There are a lot of options. I think the cheapest would be to drive to Hirtshals (Denmark) and take the Color Line ferry to Kristiansand (or Larvik), a 3 (or 3,5) hour crossing, but of course you have to drive all the way up to the top of Denmark for that, which is about 365 km from the German border.
If you want to start your trip through Norway in Oslo instead, you can take the Go Nordic ferry (with overnight stay) from Frederikshavn (Denmark) to Oslo (10 hour crossing), with a 360 km drive from the German border as well. This is currently still available through DFDS Ferries, but I haven't found a way to book there for later in the year.
Another option would the overnight Stena Line ferry from Kiel to Göteborg (Sweden) (14,5 hour crossing + a 4-hour drive to Oslo from there), while the most luxurious and comfortable option is probably the Color Line cruise ferry from Kiel directly to Oslo (a 20 hour crossing).