Abstract <p>The article presents an overview of alien fish species living in the Oka River of the Volga basin. A brief description of the Oka River is given as a habitat for fish species that are not typical of the native ichthyofauna. Information on the history of the appearance of these species, the peculiarities of biology, the occurrence in catches, and the fishery significance at different river sections located in the upper, middle, and lower reaches of the Oka River within the borders of Oryol, Tula, Kaluga, Moscow, Ryazan, and Vladimir Regions of Russia are given. The main vectors of invasions of aquatic organisms into the Oka River system for each alien species recorded as a result of ichthyological monitoring in this watercourse are shown. The forecast of the dynamics of the number of alien fish species depending on the characteristics of reproductive biology, the volume of stocking, and the intensity of fishing in the Oka River is given.</p>