re: Construction/ Home question. Help with fireplace/mold smell situation

Sounds like it could be either. Won’t know until you explore. I’m dealing with this from a shower that was leaking into my master closet. Looks like the wife is getting a new bathroom out of it.

A previous house I had was open above the fireplace box and I was able to climb down there with a ladder and run wires for the tv. You should be able to pull or cut that osb and get down there to see what might be wet. Otherwise you need to get behind that cabinet in the bathroom.