The unconventional superconductivity in the newly discovered iron-based superconductors is intimately related to its multiband/multiorbital nature. Here we report the comprehensive orbital characters of the low-energy three-dimensional electronic structure in BaFe_<1.85>Co_<0.15>As_2 by studying the polarization and photon-energy dependence of angle-resolved photoemission data. While the distributions of the d_<xz>, d_<yz>, and d_<3z^2-r^2> orbitals agree with the prediction of density functional theory, those of the d_<xy> and d_<x^2-y^2> orbitals show remarkable disagreement with theory. Our results point out the inadequacy of the existing band structure calculations and, more importantly, provide a foundation for constructing the correct microscopic model of iron pnictides.