How do I use об in a sentence?

Does it always mean 'about' ?
Is it always followed by the 6th case?

Thanks!

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I'm afraid that we really needa context here :/
the most common usage is like this:
я не хочу говорить об этом
я не хочу думать об этом
давайте порассуждаем об этом
I can't think of a case where об was followed by something else then "этом" , maybe I'm wrong.
so you can learn this collocation.
and remember that there is "a family" of prepositions here:
о, об, обо :)
it sounds very weird, I know.
but if you use some concrete pronoun or noun, like he, she, apple, house, you should use "о"/"об" depending of which is the first letter if the following word:
я мечтаю о доме
я мечтаю об Анне
я думаю о ней
он читает об истории Франции

"обо" is most commonly used with general pronoun "все"
говорить обо всем

but also with pronoun of the 1st person singular
они говорили обо мне
они говорили о тебе
они говорили о ней
они гоаорили о них
они говорили о нас

it seems that rules here are not very general :/