This paper reports the effect of KOH molarity variation on structure, morphology, and optical properties of CuO nanostructures synthesized by ultrasonic-assisted precipitation method. The results of the XRD analysis showed that the CuO samples had a monoclinic structure with lattice constants which decreased with increasing KOH molarity in the sonochemical precursors. The average crystallite size (ACS) of the CuO samples depend on the KOH molarity, which tends to decrease (in the range 16-23 nm) with increasing KOH molarity. Small ACS indicates that the CuO samples tend to have a nanocrystal structure. The morphology of the CuO samples is also influenced by the KOH molarity, which the flower-shaped CuO nanostructure was found for low molarity (0.25 M) of KOH in the sonochemical precursor. The optical characteristics of CuO samples strongly absorb the wide spectrum of electromagnetic waves below 650 nm. The bandgap energy of the CuO samples has a fairly large value and fluctuates in the range of 1.74 - 1.82 eV which varies with the molarity of KOH in the precursor solutions.