# Why oxygen is enclosed in parenthesis in some chemical reaction?

While going through my chemistry text book I came through the following reaction.

$\ce{RCH2OH + (O) -> RCHO + H2O}$

In the above reaction why oxygen atom is enclosed in parenthesis ?

It represents oxygen from an oxidising agent, as opposed to from diatomic oxygen. It's normally done in square brackets like this: $$\ce{CH3CH2OH + 2[O] → CH3COOH + H2O}$$ Is the reaction between ethanol and potassium manganate (VII) to form ethanoic acid. The oxygen in the reaction is from the permanganate, not from diatomic oxygen. It kind of simplifies the reaction, so you only see the reaction of what you want -the ethanol and oxygen - instead of the whole reaction with the manganate there. (Manganate is an oxidising agent).
In your reaction the alcohol: $\ce{RCH2OH}$ is reduced by the oxygen to form an aldehyde and water, using oxygen from an oxidising agent - the [O].
$\ce{(O)}$ means nascent oxygen. When you use an oxidising agent, fresh Oxygen is produced which is called nascent oxygen.