You'll need to explain how a slight increase in O2 will allow the engine to develop more power if there is no increase in fuel quantity...........??

A diesel already has much more air than it can possibly use in all operating modes except maximum power. At the max. power point, the fuel delivered should just barely match the available air.

Naturally, since combustion of the available air is not perfect when the air is limited (and it is limited when the available fuel just matches it), the engine will produce slightly more power and the ET might be slightly less.........because all the fuel can now be combusted.

But, this effect is slight, and certainly will not apply under part "throttle" conditions.