The extra oxygen that the o2 sensor reads with HHO being injected, in NOT from the oxygen being produced by the reactor! That tiny amount of oxygen in insignificant compared to the oxygen being brought in through the air intake.
The increase in the O2 reading is because the burn is no much cleaner because of thehydrogen burn, there are fewer contaminates -- therefore more room for atmospheric oxygen in the exhaust stream. This is exactly what the O2 sensor would see if the fuel air mixture was too lean -- less unburnt fuel making more room for the atmospheric oxygen.
1991 Plymouth Acclaim 3L V6.
1 dry cells with nineteen 6"x8" 316L ss plates, driven by constant current PWM set at 35 amps (13.3V at PWM). 28% KOH electrolyte. Total measured output 2.5 lpm. Mileage went from 18 to 26 mpg, all city driving (44% increase). EFIE set at .370 and I still need to play with ignition timing.