In the equation C4H9OH + 2[O] ---- CH3CH2CH2COOH + H2O
What does it mean by 2[O]
[O] just means oxidising agent or oxidation. The 2 is pretty meaningless, except to "balance" the equation. It is just a way of representing an oxidising agent that provides two atoms of oxygen for the reaction.
It could also mean that the primary alcohol butanol has to undergone two oxidations, the first to form an aldehyde and then the oxidation of the aldehyde to form the carboxylic acid.