Which one?

What about Handles of the door?
Why I feel there is nothing wrong when I omit "the"?

The simplest and, in my opinion, most common is 'door handle'.

Whether or not we have an article with this depends entirely on context:

I must buy a new (door) handle for the kitchen door.
Door handles are ridiculously expensive these days.
I am going to replace all the door handles in my flat.

Thank you fivejedjon,

I guess that all mean the same, but could you please provide some reasons which one is better?
(assumption: I'm referring to a known door and its handles)

What I want to find out is the way of correct usage of sequenced nouns in a row.

The door's handles
The door handles
The handles o

As I said, it depends on context - .and that means at least a complete sentence, not just a phrase.

Having said that, I cannot think of a context in which any of your phrases would be acceptable. We are just not very likely to want to speak of all the door handles of all the rooms in all the buildings in a university.

Hmm... Thanks, but what I assumed was that all the samples mean the same. So let's make it crystal clear, they have NOT the same meaning. But honestly, I didn't understand what are the differences between them. I have no clue why they have different meanings. All of them indicate that both handles and door are known, and if I read them in different contexts, I wouldn't see any differ

This is what I mean by context:

The door handles in my flat are very old.
The handles of the doors in my flat are very old.
(?) My flat's door handles are very old

Thank you fivejedjon,
Still, I see no differences in the meaning. Could you please give me some example that they mean differently?

fivejedjon(?) My flat's door handles are very old

And what do you mean from "(?)"?

PaeezStill, I see no differences in the meaning. Could you please give me some example that they mean differently?

There is no difference in the meaning of my three sentences. The only way we can give different
meanings is to say something different:

I have bought some new handles for the doors in my flat.

PaeezAnd

So you agree that all of my examples mean the same?

The door's handles
The door handles
The handles of the door
Handles of the door

What about these?

The university door's handles
The university door handles
The handles of the university door
Handles of the university door

PaeezSo you agree that all of my examples mean the same?

No, I did not say that at all.

PaeezThe door's handlesThe door handlesThe handles of the doorHandles of the door

Whether any of these can be used depends on the context. For example, in I must buy (some) new X, none of yours will work. There we could use only