\(\overrightarrow {OA} + \overrightarrow {OB} + \overrightarrow {OC} + \overrightarrow {OD} \)\( = \left( {\overrightarrow {OA} + \overrightarrow {OC} } \right) + \left( {\overrightarrow {OB} + \overrightarrow {OD} } \right)\) \( = \overrightarrow 0 + \overrightarrow 0 = \overrightarrow 0 \).
