If $OACB$ is a parallelogram with $\overrightarrow{OC} = \overrightarrow{a} $ and $\overrightarrow{AB} = \overrightarrow{b}$, then $\overrightarrow{OA}$ is equal to :

Question

( A ) $\frac{1}{2} (\overrightarrow{a} - \overrightarrow{b})$
( B ) $ (\overrightarrow{a} - \overrightarrow{b})$
( C ) $\frac{1}{2} (\overrightarrow{b} - \overrightarrow{a})$
( D ) $ (\overrightarrow{a} + \overrightarrow{b})$