Alkali and alkaline earth metal hydroxides are the most common strong bases. These bases release hydroxide groups into the aqueous medium, responsible for their basicity. Since the hydroxide ions generated by the autoionization of water are negligible compared to those generated by the base (except when the base is very dilute), we can consider that the base is the only source of hydroxide ions in the medium, and therefore both $[OH^-]=[base]$, where $pOH=-log[OH^-]$ and $pH=14-pOH$
Let's see an example: Calculate the pH of an aqueous solution of 0.1 M sodium hydroxide.
$NaOH \rightarrow Na^++ OH^-$
$[OH^-]=[NaOH]=0.1\;M$
$pOH=-log(0.1)=1$
$pH=14-pOH=13$