We could apply the same thing to your smartphone: if you were taking pictures of some view and you happened to catch something on camera which you didn't reveal to the police, wouldn't that be obstruction of justice as well?
And for that to carry any weight, the police should have to show that you had that information in the first place. If they knew you were filming a crime and you said you saw nothing, that should not be obstruction. It's just suspicious. It should be grounds for a warrant, which could be used to establish that you had captured the evidence and lied. Then it's obstruction.
As a third party witness it is different, at least in Norway.
> In general, anyone is required to appear before the courts and give testimony, except dictated otherwise by the law
Nonetheless:
> §123: A witness may refuse to answer questions leading to self-incrimination either for the witness itself, or for anyone related to the witness as described by §122
A person does not have to testify against oneself. This principle stands in the USA and Finland for sure (that I know of), I'd expect it to stand in Norway as well.