Use this tutorial for OH 2.3 and mqtt
https://www.smarthomeblog.net/diy-home-automation-sensors/
Was working for me

Try to add

#define MY_NODE_ID 2

(or another node number) in your dallas temperature sketch BEFORE including the mysensors library

@atzohy said in Some questions before I start ordering parts.:

Cannot automatic discover thing node

I'm not using the mysensors binding in openhab, i followed this https://www.smarthomeblog.net/diy-home-automation-sensors/ excellent tutorial and everything is working.

I didnt realise, that the warning was in the openhab log. Have you tried to define the node id in the sketch and add the sensor manually in openhab via a .thing and .item file like here described?
https://github.com/tobof/openhab2-addons/wiki/Examples