This is fairly easy (though it’s all relative!) - when a Tasmota device receives a command from anything it will publish a response on the RESULT topic. At the very least, that RESULT message will include the POWER state.
OpenHAB can subscribe to these RESULT topics using the stateTopic property of a thing, which means it is ‘constantly listening’ for them. Once received, you can then update your item with the required information.
If you show a screenshot of your Tasmota MQTT setup page we could probably guide you through setting up the things, items and sitemap required.
I’m currently on the phone though, so maybe see if the following link helps? It’s for an RGB LED strip, but the controller has Tasmota on it…