Particles with size in the range of 1-100 nm, considered in any dimension, are called nanoparticles (NPs). Among various nanoparticles, nickel oxide nanoparticles (NNPs) are important because of their physical, chemical, and biological properties. In the past two decades, significant research has been done on the synthesis of NNPs. Their applications range from energy storage to catalysis to antifungal and antibacterial activity. This review provides a brief overview of various methods used for synthesizing NNPs, their properties, and their applications. The synthesis methods are classified into physical synthesis, chemical synthesis, and green synthesis involving plants and microorganisms, especially fungi. Particular focus is given to the types of precursors used for their synthesis, morphology, particle size, and applications.