What is the difference between о and про?

Question

For example you can talk, read or write about a topic, and sometimes you see о теме and sometimes про тему. Is there a conceptual difference?

Answer 1

 

Besides O being more common/general and ПРО being more colloquial, these prepositions are governed by different cases.

про (+ accusative case) = about, concerning
о (+ prepositional case) = about, of, on

I assume you are trying to compare О and ПРО in the meaning of "about" only, because they have other meanings that don't cross.

о (+ accusative case) = against, upon: Опираться о стену. (To lean against the wall.)
о (+ prepositional case) = of, on: заботиться о ком-л — take care of smb.
про = на: Про всякий случай. (Just in case.)
про = для: Это не про вас. (This is not for you.)

There are also a few set phrases where only one preposition can be used but not another:

про себя (думать, читать) — (to think, read) to oneself
палка о двух концах — a double-edged weapon
бок о бок — side by side
идти рука об руку — go hand in hand

Also see: Examples of usage for preposition 'O'

Answer 2

О and про are same in the most of cases.

But there are few differences:

• О is more formal, про is more for speaking.
• О is about more global things then про. It's not beautiful to say: Мы говорили про Землю. (the Earth). The Earth is global thing so we use о.

And we have one usual phrase "о том, что...". Here we usually use о. Мы вспомнили о том, что нужно купить молоко и хлеб.