The preposition о/об can be used with the prepositional and with the accusative case.

О/об + prep. has the meaning ‘about’ and is synonymous to the prepositions про + acc. and насчёт + gen.
The most common ones are про and о/об. They are interchangeable. Про is slightly more colloquial.
The preposition насчёт is more “indirect”. It can be translated as ‘regarding’ or ‘concerning’.
Насчёт and о/об are not always interchangeable. E.g. You can say «песня о чём-либо» or «песня про что-либо», but cannot say «песня насчёт чего-либо».

О/об + acc. has a different meaning: physical touching. It can be translated by the prepositions ‘by’, ‘against’.
Идти бок о бок — “to walk side by side”.
Удариться рукой об стол — “to hit one's hand against(?) a table” (I don’t know how to say it in English correctly).