Back in 2019 a in needed a reservation please note that trains can be reservation compulsory even if not marked like that at bahn.de/DB Navigator.As the train operated within Norway as Intercity :)

There still exist even few regionaltrains that a reservation compulsory in Europe it always depends on the train company if you need a reservation or not My suggestion Contact SJ (Swedish Rail) via Facebook or Twitter (they respond pretty fast) to get a final verified answer :)

You could check with Tågkompaniet, which is the operator. If they say you have to book, the next question to them is where to book, since their website doesn't offer that possibility and neither do sj.se and interrail.eu for this route.

Tågkompaniet (nowadays Vy) have a Swedish website, Norwegian website, email and Facebook.

If you book a normal ticket on the Norwegian website, then you have the choice between "free seating” and "seat reservation” (for an additional fee), so it looks like a reservation is not obligatory.

@Nanja, this means the app is incorrect about these trains.

Thanks a lot for your answers! 

I asked Vy via Messenger and as soon as I receive an answer, I ll let the Community know about that

Hello again, just received an answer from Vy . 

It is short but clear : 

Hi, there's no mandatary seat reservation. But you may end up with no seat at all if the train is full.

@Nanja  Do you have plans to change the information in railplanner about that route? It still says reservation required / disinformation /

Hi, thanks for checking in. We are looking into the case as we speak. The problem is that the train Oslo-Goteborg has the same trains type in and provider as Oslo-Bergen. Therefore, it is not easy to make a distinction between them. To apply it manually, we need to know the specific train numbers. We are currently looking into solutions to change the information. I'll keep you posted here. 

Addition 13/01/22:  We managed to find a solution so the Oslo- Goteborg route is now shown as reservation optional. 

@Nanja  Not sure if it helps, but let me show you screenshots from my Messenger chat with Vy .

First is my question, then I upload the same screenshots I posted here and final - their answer :