Yakşamlar! Benim adım Zhivko!

Please, help me and answer this question: what´s the difference between "O güzeldi." and "O güzel oldu."

Teşekkur ederim!

iyi akşamlar Zhivko 

Click here to read an explanation by Tunci and Scalpel

I see the small difference as It was beautiful, as opposed to It became beautiful.

O = he/she/it / that,  but you need to read the sentence before ´o´ is used, to know the exact meaning.

Cevabınız için çok teşekkür ederim, Henry!

I saw the posts by Tunci and Scalpel but there´s another question now:

Does " O güzel oldu" means that She (he, it) became beautiful suddenly, unexpected or it means that She has become beautiful (something like Present Perfect) and she still is so.

"O güzeldi" means that she was beautiful (like Simple Past) but now maybe she isn´t so.

Please Henry, am I right?

Saygılarımla!

My understanding for your first question (güzel oldu) is that they became beautiful, and still continue to be beautiful.

The second part, I would guess indicates to me that they have died, or not aged well.

I see Turks use güzelsin (you are beautiful) or O güzel(dir) (she is beautiful) when someone is still beautiful, rather than the past tense versions.

Hopefully a Turk will confirm this, or give their opinion 

Not necessarily. They don´t mean any more than the given meanings without a context, so I´d say don´t complicate things and confuse yourself. I will give you some more examples for further clarification

Sabah bir kız gördüm, çok güzeldi = I saw a girl in the morning, she was very beautiful.

O gençken güzeldi = She was beautiful when she was young.

Makyaj yaptıktan sonra güzel oldu (güzelleşti) = She became beautiful after she put on make up.

Çocukken pek güzel değildi, büyüdü güzel oldu (güzelleşti) = She wasn´t that beautiful when she was a child and she grew up and became beautiful.

Now, a trickier one:

Dün eve giderken bir kaza oldu= While I was going home yesterday, there was an accident (there happened an accident, I saw it happen)

Dün eve giderken yolda bir kaza vardı= While I was going home yesterday, there was an accident on the way. (The accident had happened earlier, I came across it)

I said tricky, because these two usually translate the same into English (i.e. There was an accident). I´m not sure if "there happened" or "she became beautiful" would be natural expressions in these examples(Perhaps Henry can clarify this for us) but you can see the nuances this way.

Şimdi anladım.

Hepinize ama özellikle gugukkusu, teşekkür ederim!