Most hearing aids, not just the OPN, starts out with an underpower fitting on purpose as not to overwhelm the patient on the first days. It can be scheduled to ramp up to full volume potential over a period of time (as determined by your hearing professional). So it is designed to adjust automatically, but not because of some smart sensor that does it on the fly or anything like that. It’s simply a 3 phase schedule, with phase 1 being the lowest power fitting, phase 2 medium power, and phase 3 full power. But the interval between these phases are programmed in by your hearing professional. People who adapt better and faster may notice this underpower issue sooner than others.
Just ask your hearing professional to do a REM test for you, and accelerate the fitting schedule for you sooner rather than later. In the mean time, just use your volume button to compensate until then, and just remember that this is definitely a fixable issue on your next visit.
You can also go the other way, ask your hearing professional to give you full power right now, and if it’s too overwhelming for you, then just use the volume button to manually dial back the loudness.