Some things to consider:
- In OBS mixer panel slide the mic to 0.0db
- Go to Windows Sound Control Panel and increase the volume level appropriate to the mic device.
- Increase gain from the mic itself using its knobs/dials (if it has any)
- Increase gain from the audio interface using its knobs/dials (if it has any)
- Disable mic/auxiliary in OBS audio. Add the mic as an OBS source in your scene and add a gain filter.
Also, if your mic is one that needs +48V phantom, you need to address that with a new mic or appropriate audio hardware.