Ikke overse problemet.

Because this is a negative command.

In Norwegian, a very common way to make a negative imperative is:

Ikke + verb

So:

• Ikke gå! = Don’t go!
• Ikke glem det. = Don’t forget it.
• Ikke overse problemet. = Don’t overlook the problem.

This is the normal, everyday pattern.

Overse is a verb: å overse.

In this sentence, it means something like:

• to overlook
• to fail to notice
• to neglect
• sometimes to disregard

So it is a single verb, not just over + see translated word-for-word from English.

It looks very similar, but you should be careful: this is a false friend.

In English, to oversee usually means to supervise.

In Norwegian, å overse usually means to overlook / fail to notice / ignore.

So even though the words look alike, they do not usually mean the same thing.

It is the imperative form, because the sentence is giving a command.

The infinitive is:

• å overse

The imperative here is:

• overse!

That may look unusual if you are expecting a shorter form, but this verb behaves like other verbs built on se:

• å se → se!
• å overse → overse!

So in this sentence, overse is the command form.

It is understood, not stated.

In Norwegian imperatives, the subject is usually omitted, just like in English:

• Sit down.
• Don’t forget.

Norwegian does the same thing:

• Kom hit.
• Ikke glem det.
• Ikke overse problemet.

The sentence can be said to one person or more than one person, depending on context.

Because problemet is the definite singular form, meaning the problem.

The noun is:

• et problem = a problem

The definite form is:

• problemet = the problem

Norwegian usually marks definiteness by adding an ending to the noun, instead of using a separate word like English the.

So:

• problem = problem
• et problem = a problem
• problemet = the problem

This is a very important difference.

• å overse = to overlook / fail to notice / ignore
• å se over = to look over / review / check

Examples:

• Ikke overse problemet. = Don’t overlook the problem.
• Kan du se over rapporten? = Can you look over/review the report?

So overse is one verb with its own meaning, while se over is a different expression.

Yes, you could, but the nuance is a little different.

• å ignorere usually suggests consciously ignoring something
• å overse can suggest either not noticing it or failing to give it proper attention

So:

• Ikke ignorer problemet = don’t deliberately ignore the problem
• Ikke overse problemet = don’t overlook the problem / don’t miss it / don’t neglect it

In many situations they are similar, but they are not always identical.

People would understand it, but Ikke overse problemet is the more natural and standard way to say it as an everyday negative command.

Putting ikke first is the usual pattern in Norwegian imperatives.

So for a learner, the safest choice is:

• Ikke overse problemet.

It is basically neutral standard Norwegian.

It works well in normal written Norwegian and would also sound natural in speech, depending on context.

It is not especially formal or especially casual. It is just a straightforward warning or instruction.