Oxidiser to fuel ratios are specified how? | Propulsion Workgroup | Forum

Mass.

Also, that's not always the optimal. LOX/H2 engines are run very fuel rich, particularly at low altitudes, because small molecules convert thermal energy into kinetic energy better than larger ones, so a mixture rich in H2 produces better nozzle efficiency. Where the optimum lies depends on the altitude, the properties of the nozzle and how much your hydrogen tank weighs. I don't have the engineering or chemistry background to figure this stuff out, unfortunately.