After recording I found out, that the audio was much, very much too loud. I could not use this audio for my video.
Is there a way to have the Osmo reduce the sensitivity of the microphone automatically?

Go to the mic setting and turn down the mic gain.

Hi Siebie, Cephas's right, please turn down the Audio Volume Control in Audio Control to have a try, thank you.

No -- there's no AGC (automatic gain control) and no limiter. If you can't pre-set or ride your gain, then you'll have to record things seperately.