\begin{align} \frac{1}{2} \left| \overrightarrow{\mathrm{OA}} \times \overrightarrow{\mathrm{OB}} \right| &= \frac{1}{2} \left| (8, 2, -6) \right| \\ &= \frac{1}{2} \cdot 2 \sqrt{26} \\ &= \sqrt{26} \end{align}
\begin{align} \frac{1}{6} \left| \left( \overrightarrow{\mathrm{OA}} \times \overrightarrow{\mathrm{OB}} \right) \cdot \overrightarrow{\mathrm{OC}} \right| &= \frac{1}{6} \cdot 50 \\ &= \frac{25}{3} \end{align}