That's what I'm saying. If you left the O2 sensor plugged in, it'll recalibrate on the fly and provide the correct mixture regardless of fuel type. The fact that you needed to remove it to get a result tends to prove that it's simply leaned out the mixture, not created a different type of combustion. I'm not sure how the HHO is actually even necessary to do this.
It'll matter if it burns the piston or valves. Detonation can literally burn a hole through a piston, which will cost far more than saving 14% on many many fillups. Unfortunately, you really can't prove whether it's at risk of detonation or not. Since you've got the system hacked rather randomly, it will not react to different fuel blends, elevations, or temps as intended. In fact, the ethanol blended fuel sold all over now is a problem for sure because the ethanol content varies. Ethanol is less dense than gasoline and requires less oxygen for a given injector pulse. In normal use, the O2 sensor adapts the system as ethanol content varies, but you've got no method of adaption for that at all.
And the other point- the smog emissions- NOx- may be tens of times worse than allowed. Hundreds, even. It's not really responsible or "green" to save a bit of fuel by spewing a lot of toxic emissions. Like I'm saying, the lean-burn fuel improvements were already known to work, but banned because of the pollution problem. Does your city do emissions testing? I mean, sure, you could just plug the O2 sensor back in for that day, but again, not really a responsible thing.
I tried the acetone-in-the-fuel thing for kicks awhile back in an old 302 5L van. Carefully used the "recommended" ratio. Mpg did not improve. Failed for low-speed NOx emissions, only time I've ever had a failure. Well, I did some random "fixes" on things (changed the air filter and spark plug wires, unlikely to make a big difference) and run that tank near empty before refilling to flush it out. Then, suddenly, it was well within normal again. Well, I WISH I could have brought it back for retesting after flushing the acetone, but you're only allowed one free retest and it's a long wait to have them do the test. I think it's pretty likely that the acetone did it.