The present study reports a simple and cost effective chemical approach for conveniently synthesizing cupric oxide nanostructures (CuO-NSs). To promote confined and stable growth of CuO-NSs, hexamine (HMT) capping molecules were used in the reaction process. Addition of capping molecule could greatly influence the size confinement of CuO-NSs which in turn affects the optical properties. Significant variations in surface morphology were attributed to the presence of hydroxide anion in HMT during synthesis process. Transmission electron microscopy (TEM) results indicated that much larger CuO-NSs are formed without using HMT. XRD analysis revealed that the sample without HMT gives higher crystallite size with less strain. Energy dispersive X-ray analysis with elemental mapping confirms the stoichiometric proportion of CuO-NSs. Micro-Raman and Fourier transform infrared spectroscopy results confirmed the effect of HMT on monoclinic structure. The optical properties of the prepared CuO-NSs were investigated using UV–visible diffuse reflectance spectroscopy and photoluminescence (PL) spectroscopy. Optical band gap energy of CuO-NSs was increased with increasing HMT concentration from half to equimolar ratio. The PL spectra of CuO-NSs revealed the presence of emission bands in violet, blue-green and IR region due to band-gap expansion presence of oxygen vacancies and carrier localization effect in the defect levels. The PL results were in good agreement with TEM observations. Controllable morphology of CuO-NSs with adjustable optical properties has shown excellent applicability in many applications such as catalysts, gas sensors and solar cells.