Coordinate geometry — practice question

Taking origin $O$, the position vectors of points $A$ and $B$ are given by $\overrightarrow{OA} = \begin{pmatrix} 3 \\ -6 \\ p \end{pmatrix}$ and $\overrightarrow{OB} = \begin{pmatrix} 2 \\ -6 \\ -7 \end{pmatrix}$, and angle $AOB = 90^\circ$. Point $C$ is defined so that $\overrightarrow{OC} = \frac{2}{3}\overrightarrow{OA}$.