The domain structure of antiferromagnetic (AFM) NiO(100) has been investigated by nonmagnetic x-ray linear dichroism (XLD) at the O K edge using photoemission electron microscopy and linearly polarized x-ray light. The evolution of XLD image contrast as the function of the angle between the crystal orientation and the incident s- or p-polarized light is clearly observed. The angular dependence of the XLD contrast is in good agreement with the calculated x-ray absorption cross section of O 1s to 2p orbital that is deformed anisotropically by the strong hybridization with Ni 3d orbitals. This agreement strongly supports the conclusion that the observed XLD contrast reflects the twin-domain (T-domain) structure of the NiO crystal. By comparing the experimental data with the calculation, it is possible to assign each domain contrast to a specific T-domain in the quantitative manner. The proposed simple and clear way of T-domain assignment will be useful for investigating the magnetic domain structures of NiO and other oxide AFM materials such as FeO, CoO, and CuO, and will be important for understanding the magnetic properties of AFM materials, such as the exchange bias effect.