ALT Toolbox paper in @conphysjournal @RasmusErn & @FredrikJutfelt https://doi.org/10.1093/conphys/coae024. This paper describes an optocoupler-based regulation apparatus for saturation manipulation of oxygen in water (OptoReg). This system enables control of solenoid valves for oxygen and nitrogen gases using a FireSting-O2 meter, an optocoupler box, and an electronic switch box. The hardware components connect to a computer through USB cables. The control software is free and has a graphical user interface, making it easy to use. With the OptoReg system, any lab with a computer running Microsoft Windows operating system and a 4-channel FireSting-O2 meter can easily and cheaply set up four independently controlled systems for regulating water oxygen levels. Here we describe how to assemble and run the OptoReg system and present a data set demonstrating the high precision and stability of the OptoReg system during static acclimation experiments and dynamic warming trials.