The stress and lattice constants in zinc oxide (ZnO) nanoparticles play a major role in determining the distortions that occur in the crystal during the preparation of the sample as a result of exposure to several factors, such as external strain, temperature, pressing and structural defects (oxygen vacancies and zinc/oxygen interstitials). ZnO nanoparticles (20 nm) were used to make high-density ZnO discs via uniaxial pressing at 4 ton cm–2 and sintering at 1200°C for 1 h. Structural, elemental and optical characterisations were then performed on the samples using various techniques. High-oxygen thermal annealing significantly affected the ZnO discs, particularly in enhancing the growth of the grain even at a low annealing temperature (450°C). The strong solid-state reaction during annealing may be attributed to the high surface area of the 20 nm ZnO nanoparticles that exhibited a strong surface reaction even at low annealing temperatures. The annealing treatment also improved the grain crystallinity, as shown by the transition of the intrinsic compressive stress to tensile stress based on the XRD lattice constant and full-width at half-maximum data. Therefore, high-oxygen thermal annealing can be used as a new technique in controlling the stress in nanoparticle-based ZnO discs and produce discs with improved structural and optical properties.