I purchased a home 6 months ago. It has a 1/2" plywood subfloor with 3/4" particleboard laid on top of it. I am now in the process of removing carpet and particle board to install 3/4" solid hardwood floors. My plan is to remove the particleboard and replace with 25/32" advantech osb. I have measured the moisture content of the osb and it is 0%. The moisture content of the 1/2" plywood subfloor is mostly 8%. However along the exterior walls I have had readings much higher 15-19%. The partcleboard I removed along these exterior walls was not wet.

Am I okay to lay the advantech osb and continue to put the hard woods down?

Most meters only read down to 6%. I'd let it acclimate in the house first - sticker stacked so airflow can get around the sheets.
Not sure whats happening along the walls. Do you have a crawl space?

No crawl space. However it is over a basement. There is a deck that ties into the house on the exterior of the wall.

Check to make sure the deck has flashing where the deck ledger board is attached to the exterior wall. If no leaks, you should be OK. If it has leaked, you should see stains on the sub-floor.