I´m trying to understand something. 

Oturma odası = living room

Oturma odasında = in the living room

Why isn´t it odasıda? Is the n added because of the vowel?

Thank you

n is a buffer letter

you use n as buffer:

- for possessor endings

- with a word which already has a possessed endings

- with the personal pronouns bu, şu, o

- with the - ki ending

look here

http://www.turkishclass.com/turkish_lesson_126

That is called Pronominal "n" [Zamir "n"si] officially which I don´t agree with the name. I prefer calling it " buffer n " [Kaynaştırma n´si ]. Since it does function as a buffer element  in a word. 

It is seen to be used in the following  cases ;

1. When a pronoun takes a case ending ;

O  [he , she ,it]  + n  +   a   ------>  Ona   --->  to him/ to her / to it

 " O"  is a personal pronoun.

Bu ---->  this 

Bu + n  + u ---> Here "bu" takes accusative marking.

Bunu   biliyorum  ---->  I know this. 

" Bu" is demonstrative  pronoun.

* My objection to the name "Pronominal n"  is because ;

1. It is not only used for pronouns [ it is also used in compound structures after the possessive suffix]

2. It does not work with question pronouns [ Soru Zamirleri] , such as "Nere"

Nere = where 

Nere + y + e  ---->  to where 

Nereye  baksam hep seni hatırlıyorum. ----> Whereever I look, I remember you. [ Everwhere I look reminds me of you ]"

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2.  When the compounds takes case marking , it comes between the possessive marking and case marking.

Ali´nin  kitabı --> Ali´s book  

            kitap + ı [possesive marking which indicates that the book is possessed by Ali]

Ali´nin  kitabı + n + ı [ accusative marking] 

Ali´nin kitabını  aldım. ----> I took Ali´s book.

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3.  It comes before the genitive marking , if  the word ends with a vowel.

Oda ---> room ---> this word ends with a vowel "a"

If we want to say  " the room´s door" [ the door of the room] we need to put the genitive marking which is " ın "  and we buffer with  " n"  in between them.

Oda + n + ın  --->  The door´s 

Odanın  kapısı --->  The room´s door [ The door of the room].

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4. When the word takes case ending after "ki" relative pronoun ;

Masa = table

Masada  = on the table 

Masada + ki   =  the one [that is] on the table 

Masada + ki +  n + i [accusative case ending] 

Masadakini  ver  ---> Give [me] the one [that] is on the table.

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In your example ;  

Oturma  odası [ living room] is a compound structure,  " the room of  living "

After the possessive marking "ı" , the locative case marking " da" [in] comes and we need " n " to combine them. 

Oturma  odası + n + da ==> In [the] living room [ in the room of  living]

And  YES, technically  it is because of vowel , the buffer "n" is added.

Yes it is not very clear at first sight why this naming is used and "a buffer" would do the same job I guess.

But I still think there is a very sophisticated reason to call this buffer a pronominal -n-. I thought about your reasons a bit.

1. Can we really say the compound use is not pronominal as well? Pronominal -n- is always attached to a possessive suffix. Adding a POSS is a typical way of creating pronouns out of nouns and adjectives in Turkish (başka > başkası, bütün > bütünü, bir > biri etc.). Maybe what we see here is a widening of this rule.

2. I think it is natural that there is no pronominal -n- between nere and a case ending. nere is a noun and it means ´which place´ even though it has been derived from the question pronoun ne. (I wonder if it is ever used independently though...) I mean it is not a very good example. Instead I would ask why ne does not take a pronominal -n- but a buffer -y- in its declinated forms like neyi, neye  -  maybe *neni, *nene would not sound as beautiful. (You people don´t say *göl susu either even though it would be more logical.)

And, after all, adding a possessive suffix to ne makes it act regularly again like any pronoun: Onun ne|si|n|e aşıksın?

Nere  can be used independently just like  ´bura ´.

Bura  nere , diye sordu.  ----> " Where is here? "  he asked. 

and the answer is  ; 

Bura  Amasya. ---->  Here is Amasya. 

So, from  this point of view it can be classified as "question pronoun´ as it is asking the name of the place. 

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"Ne" , very clearly is  a question pronoun, but yet  it doesn´t take "n" . Rarely you can hear people use  " Nene"   instead of " Neyine" for example in the phrase below ;

1.[Senin] Nene  lazım. !

2. [Senin] Neyine  lazım [gerek] ! ---> What´s it to do with you ? , It doesn´t concern you, it is not your problem. …etc.

[Senin] Neyine  lazım [gerek]  =  [Senin] Nene  lazım

At the end of the day, both ways are in possessive form. In the first form " ne " is buffered with "n" [ rare use], whereas in the second form  "ne" is buffered with " y" [common use].

" Su" is an interesting word.

In Archaic Turkish it was called ´ sub´ , later it changed into ´suv´.

First, –b > -w alteration [ in Uygur Turkish] and later the  -w > -v   alteration occurred [single v , in Çağatay and Kıpçak Turkish].  

The verb "suvarmak" is derived of this root. 

suvarmak = to give water to animals. To make animals drink water.

As in the anonymous folk song below ; 

Bülbülü  suvardım altın tasla

Gurbete yolladım kara yasınan.

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But  in time, the "v" sound dissolved and it got its latest form as just " SU " . 

My theory is ;

Because of the  initial "v" sound was still subconsciously strong in people´s mind, they didn´t adapt it into possessive form , instead, they kept the "v" as " y" . And automatically applied it to possessive forms, in other words, they didn´t feel to put "n" or "s".

I blame "v" for that. 

Just a theory..no more.