  We report on $^{75}$As-NMR measurements in single crystalline
BaFe$_{2}$(As$_{0.96}$P$_{0.04}$)$_{2}$ for magnetic fields parallel to the
orthorhombic $[110]_{\rm o}$ and $[100]_{\rm o}$ directions above the
structural transition temperature $T_{\rm S}\simeq121$ K. A large difference in
the linewidth between the two field directions reveals in-plane anisotropy of
the electric field gradient, even in the tetragonal phase. This provides
microscopic evidence of population imbalance between As-$4p_{x}$ and $4p_{y}$
orbitals, which reaches $|n_x-n_y|/|n_x+n_y| \sim 15$\% at $T\rightarrow T_{\rm
S}$ and is a natural consequence of the orbital ordering of Fe-3$d_{xz}$ and
$d_{yz}$ electrons. Surprisingly, this orbital polarization is found to be
already static near room temperature, suggesting that it arises from the
pinning of anisotropic orbital fluctuations by disorder. The effect is found to
be stronger below $\sim 160$ K, which coincides with the appearance of
nematicity in previous torque and photoemission measurements. These results
impose strong constraints on microscopic models of the nematic state.
