Bu odada tuhaf bir koku var.

Because -da is the locative case ending, which means in / at / inside.

• oda = room
• odada = in the room

So Bu odada means in this room.

Bu means this.

• bu oda = this room
• bu odada = in this room

In Turkish, bu comes before the noun, just like this in English.

This is because of vowel harmony and consonant harmony.

The locative ending has four forms:

• -da
• -de
• -ta
• -te

For oda:

• the last vowel is a, so the ending uses a rather than e
• the word ends in a voiced consonant-like sound pattern, so it takes d rather than t

So:

• oda + da = odada

In Turkish, when an adjective comes before a noun, bir often goes between the adjective and the noun.

So:

• tuhaf bir koku = a strange smell

This is a very common pattern:

• güzel bir ev = a beautiful house
• uzun bir yol = a long road
• ilginç bir fikir = an interesting idea

So tuhaf bir koku sounds natural and standard.

Here, bir means a/an, not specifically one.

So:

• bir koku = a smell
• tuhaf bir koku = a strange smell

Depending on context, bir can also mean one, but in this sentence it is just marking an indefinite noun, like English a.

Var means there is / there exists / is present.

So the whole sentence is literally:

• Bu odada = in this room
• tuhaf bir koku = a strange smell
• var = there is

Literal structure: In this room, there is a strange smell.

Turkish often uses var to say that something exists somewhere.

Because Turkish is expressing the idea as there is a strange smell in this room, not as this room smells strange.

English can say both:

• There is a strange smell in this room.
• This room smells strange.

Turkish can also express both ideas, but this sentence uses the existence pattern with var.

A more direct smells version would be something like:

• Bu oda tuhaf kokuyor. = This room smells strange.

So the sentence you have is not wrong or incomplete—it is just using a different structure.

Grammatically, the thing that exists is tuhaf bir koku.

So in a practical sense, the sentence is about a strange smell being present in this room.

Turkish does not need a dummy subject like English there in there is.
English says:

• There is a strange smell...

Turkish simply says:

• Tuhaf bir koku var.

So var works without any equivalent of English there.

The most neutral word order here is:

• Bu odada tuhaf bir koku var.

But Turkish word order is fairly flexible, and changing it can change the emphasis.

For example:

• Tuhaf bir koku var bu odada.
  = There’s a strange smell in this room.
  (More emphasis on a strange smell)

• Bu odada var tuhaf bir koku.
  This is much less natural in normal speech.

So yes, word order can change, but the given sentence is the most natural standard version.

Yes, you may hear bir tuhaf koku, but tuhaf bir koku is the more standard and common pattern for learners to use.

Compare:

• tuhaf bir koku = a strange smell
• bir tuhaf koku = also possible, but less neutral in many contexts

As a learner, tuhaf bir koku is the safest choice.

Not necessarily. Koku is a general word for smell / odor / scent.

Whether it is good, bad, or strange depends on the context or on an adjective:

• güzel koku = nice smell
• kötü koku = bad smell
• tuhaf koku = strange smell

So koku itself is neutral.

Because apostrophes in Turkish are generally used with proper nouns, not ordinary nouns.

So:

• Ankara’da = in Ankara
• Ahmet’te = at Ahmet’s / with Ahmet

But with a common noun like oda, you just attach the ending directly:

• odada

So oda’da would be incorrect.

You would replace var with yok.

• Bu odada tuhaf bir koku yok. = There is no strange smell in this room.

This is a very common pair in Turkish:

• var = there is / exists
• yok = there is not / does not exist

They are similar, but not exactly the same.

• Bu odada tuhaf bir koku var.
  = There is a strange smell in this room.
  Focus: a smell exists in the room.

• Bu oda tuhaf kokuyor.
  = This room smells strange.
  Focus: the room itself smells strange.

In many situations they can be used similarly, but the first one points more to the presence of a smell, while the second describes the room’s smell more directly.