take a look at Rhasspy
or
https://sepia-framework.github.io/
or
https://mycroft.ai/
Rhasspy probably offers the biggest flexibility but you will have to build your own integration into openhab over something like nodered or mqtt.
Sepias developer is active here on the forum and i think he build some kind of openhab integration into it. As i have no experience with sepia best ask him yourself here:
Mycroft is not completely offline yet as it works best with google stt but there working on transitioning to mozilla deepspeech and they have a very active community of makers and an openhab skill.
I think those three are your best contenders.
If you got lots of time and want a project you can build your own too.
-There is snowboy as a hotword engine.
-sox and its integrated silence detection to record and do vad
-pocketsphinx python for stt (you will have to build your own domain specific language model to make it work good)
-for nlu you can just start with something like simple regex matching
-tts there is pico2wav
-tie it together with something like nodered
Best regards Johannes