how to tag Organisasjonsnummer (organization number)

I guess it is really country dependent.

It looks like SIRET in France, for which we have a dedicated tag : ref:FR:SIRET.

In taginfo you'll see that there is 3 instance of the tag organisasjonsnummer already. But maybe it would be best to use namespace like ref:NO:organisasjonsnummer to make to meaning clear to foreign data users.

Anyhow, I think this would need discussion on a local communication channel, and documentation in the wiki ! ;-)

Best regards.

There's a developing consensus to use the format suggested by @H_mlet for country specific references, i.e. "ref":<country_code>:<description of ref/number>. Recent examples, which were discussed in detail on mailing lists include ref:GB:uprn and ref:GB:usrn. The advantage of this approach is that it identifies identifiers which are of national interest only, and avoids potential clashes where the same abbreviation is used in different countries.