Answer
$I= 1.94\times10^{-46}kg \cdot m^2$
Work Step by Step
Treat the diatomic oxygen molecule as 2 point masses, separated from the axis of rotation by half a bond length. Calculate the rotational inertia.
$$I= \Sigma mr^2=2mr^2$$
$$I=2((16u)(1.66\times10^{-27}kg/u))(6.05\times10^{-11}m)^2$$
$$I= 1.94\times10^{-46}kg \cdot m^2$$
