Direct experimental evidence for dissociative photoionization of oxygen molecule via the (2)Sigma(-)(u) ionic optical dark state is presented by an investigation using the method of threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Besides vibrational progress of the B-2 Sigma(-)(g) state, several weak vibrational bands of the (2)Sigma(-)(u) ionic optical dark state are observed concomitantly in an excitation energy range of 20.2-21.1 eV. Only O+ fragments are detected in the whole excitation energy range; therefore, all vibrational bands are completely predissociative. TPEPICO three-dimensional time-sliced velocity images of O+ fragments dissociated from vibrational state-selected O-2(+)((2)Sigma(-)(u),v(+)) ions are recorded. For the (2)Sigma(-)(u)(v(+)=0-3) vibrational states, only the lowest dissociation channel of O+(S-4) + O(P-3) is observed. Once the photon energy is slightly increased to the (2)Sigma(-)(u)(v(+)=4) level, a new concentric doughnut appears in the image, indicating that the second dissociation channel of O+(4S) + O(D-1) is identified indeed. With the aid of potential energy curves, the dissociative mechanism of O-2(+) in the (2)Sigma(-)(u)(v(+)) state is proposed.