There simply is no dictionary yet for Norwegian. But you can use this tool to create a dictionary. You need a wordlist, as described here. The output .dict file must be put in res/raw.

It seems like this repo has a Norwegian wordlist which is the same as AnySoftKeyboard's.

So it only needs to be converted to that raw format?

I thought the language code for Norwegian was no but you're right that the nb file is indeed the correct dictionary. So you're probably right that it just needs to be converted.

I have an apk build with Norwegian (bokmål) wordlist / dictionary that works fairly well for me anyway, Dunno what to do with it on here tho cos it's just OpenBoard v1.4.5 with Norwegian spellcheck stuffed in it, so it's not like it's my app exactly. Only tried Norwegian, Swedish, English (UK) and Spanish, but they all seem to work well enough.