This study investigates the impact of a Pt underlayer on the antiferromagnetic properties of epitaxial NiO(001) films, focusing on the antiferromagnetic domain structure and interfacial exchange coupling. Two types of epitaxial NiO films were prepared: one directly deposited on an MgO(001) substrate and the other with a 10-nm-thick Pt layer inserted between the NiO film and the MgO substrate. Despite both films exhibiting epitaxial growth, detailed structural analyses reveal that the insertion of Pt underlayer leads to the strain relaxation and introduces the misfit dislocations at the Pt/NiO interface. Moreover, the exchange bias and coercive fields of the adjacent CoFeB layer are significantly enhanced in the sample with the Pt underlayer. X-ray magnetic linear dichroism photoemission electron microscopy (XMLD-PEEM) directly visualizes a drastic reduction in the antiferromagnetic domain size by the Pt insertion. The disappearance of micrometer-scale XMLD contrast and the emergence of an in-plane Néel vector orientation indicate that the strain relaxation and interfacial disorder introduced by the Pt underlayer strongly modify the antiferromagnetic domain configuration. These results demonstrate that the Pt underlayer serves as an effective structural and magnetic control parameter in epitaxial NiO, providing a pathway to tailor antiferromagnetic domain structures and interfacial exchange coupling for NiO-based spintronic devices.