Organic reactions are typically carried out in the presence of solvents. Isolation of the pure products requires separation and purification steps, which result in a substantial decrease in yield and can be environmentally hazardous processes. A simple and efficient way to increase yields and reduce environmental impact is to conduct the reaction in the absence of solvent, which includes solvent free or solid-state reactions. Solvent-free organic reactions have drawn great interest, particularly from the viewpoint of green chemistry, and environmentally friendly solvent-free reactions have been investigated widely. Due to enormous advantages of solvent free reactions, various solvent free approaches are being discovered for ecofriendly synthesis of many compounds. Metal oxide nanoparticles in the form of nanocatalyst have emerged as viable alternatives to conventional materials in various fields of chemistry and attracted marvelous interest of chemists. This is because the activity of the catalyst resides in the exposed portion of the particles by decreasing the size of the catalyst, advantages such as more surface area would be exposed to the reactant, only negligible amount would be required to give the significant result and selectivity could be achieved, thereby, eliminating the undesired products. The nanocatalyst is inexpensive, stable, can be easily recycled and reused for several cycles with consistent activity. The current review enlists the various types of transition metal oxide nanoparticles involved in catalysis of the organic synthesis.