Micro-nano bubble (MNB) technology is becoming an increasingly important research direction due to its unique physicochemical properties and wide range of industrial applications. Micro- and nano-scale gas bubbles dispersed in liquids are characterized by a large interfacial area, long residence time in water, high mass transfer efficiency, and the ability to generate reactive species during implosion processes. These properties create significant opportunities for improving the efficiency of water treatment and energy systems. Microbubble generation by hydrodynamic cavitation depends strongly on both the geometry of the cavitation device and the operating conditions of the system. This study presents an experimental optimization of a laboratory-scale hydrodynamic bubble generator, with particular attention to cavitation-nozzle diameter, water flow rate, and nozzle/air-inlet location. Dissolved oxygen concentration was used as an indirect indicator of bubble generation and was compared with the temperature-dependent oxygen solubility in water. The results showed that the tested hydraulic configuration significantly influenced the observed oxygen response and the formation of dispersed gas structures. Microscopic observations indicated that the generated bubbles were in the micrometre range and exhibited limited stability after sampling. The study demonstrates that nozzle geometry, flow conditions, and component arrangement are important parameters affecting the performance of hydrodynamic microbubble generators and should be considered in further system optimization.