All stories are in 3rd person omniscient, whether you like it or not. Some stories look as if they are, and they are. Some stories look as if they are not, and they are.
So what does this mean?
I make this statement from time to time, and it's based more on a hunch than (thus far) a cogent argument. So rather than my usual rambling edifice, I'm going to try to explain why I have this hunch, or possibly instinct.
Firstly, a story needs someone to tell it. A real person. Whether he writes on the page or recounts to the room, he is a real, physical presence.
If a story has been made up, it must have been made up by someone. Since it comes from their imagination, they must know everything about the story, because anything they don't know, they can make up. Hence omniscient. The writer is always omniscient.
Non-omniscient points of view are a technique for restricting the flow of information from the writer to the reader, but the writer is always omniscient.
Can I tell a story where I'm not omniscient? If I told a true story, where I told my side of the story, based on what I knew, I suppose it would be 1st person restricted, but I doubt whether a POV definition is really meaningful in such a case. True stories become memoires, anecdotes and even biographies, because someone has, after the event, decided to present the event as a story. In such cases the writer knows more than he did at the time - if nothing else, he knows how the story will end, which he didn't know when the event was happening.
Which suggests to me that the writer is always omniscient.
But I also have this instinct that even POV is an illusion. 1st (or 2nd) person POVs are a sort of frame for 3rd person.
Now Robinson Crusoe is written in first person. Initially this presents me with a difficulty; one might suppose that storytelling arises from a need to communicate the events of one's life to others, and hence 1st person comes first. But a translation inevitably occurs, even in the mind of the teller: I am telling you a story about something that happened to me. Not me now, but me then. Every story must have an object, and that object is about me.
The main character of a story, then, is that story's object, the accusative of the story that is about him. That doesn't imply a 3rd person, but something does.
I think that something is related to the fact of knowing the ending; any telling, any bearing witness, becomes a story once the witness knows the end. Once the beginning, the middle, and the end become matters of the teller's choice, he will choose them according to his ideal of a good story. This encapsulation transforms real past events into a story. And this transformation into a discrete idea, "a story", separates the actors in the story from the continuum of their daily lives. Their story-self becomes someone else: a character. Once this story-self is created, the story can be passed on wholesale to another teller, who, knowing that it is not about him, may now chuse to tell it in the 3rd person, without changing the story.
I contend, indeed, that the "first person true story" only has a special effect on those who know personally the teller; for everyone else, it is a story whose truth is judged not on the claim of truth, but on the same standard as all "made up" stories: verisimilitude, or the appearance of truth. Hence the main character, the actor narrator, is for those who do not know him, a 3rd person.
To summarize, here, then, is what I think happens:
The writer knows all about the story, because he knows when it starts, he knows what happens throughout, and he knows how it ends, even at the beginning. The writer also knows who the story is about. The object of the story is a person that the writer has chosen to be the object. For me, these two facts make any story 3rd person omniscient.
The writer can them simply start writing.
Or he can chuse, for a variety of literary or strategic or even marketing purposes, to add an extra layer, the "imaginary narrator". He might, like Watson, know the whole story but not understand all of it, or be a POV that is really no more than a POV, like a man piecing together a story from security camera footage. I imagine the choices must be pretty broad, and many of them probably add a great deal to the variety and intensity of the reader's experience.
But behind all of them is the archetype of the story being told. And the archetype, the platonic ideal of any story, is related in the past tense, in the 3rd person, by an all knowing narrator*.
* at least, I think so. That's how it looks to me. But I wonder, Wittgensteinlike, how it would look to me if it looked as if it was all present tense first person restricted? I think I need to go lie down.