Yes, you can use Livereacting with OBS in a plugin mode. Check out how to use OBS plugin here.
As an alternative, you can use the following method as well, but integrating the plugin would be faster and more convenient.
The alternative method of connecting OBS:
1. Set your source
2. Open Mixer, and select the Advanced Audio Properties
3. Enable only your microphone for monitor. You should hear yourself in the headphones after enabled
4. Launch the OBS Virtual Camera (Tools > Virtual Cam)
5. DO NOT click "Start Streaming" in OBS
Once you set it up, you connect OBS camera as web camera in LiveReacting studio.