So I've begun pulling the carpet up to put down solid 3/4" oak flooring. I'd previously thought I needed to pull the particle board out and I'm trying to figure out if I still need to. I have now found out I have

1/2" plywood sub / layer of tar paper / 5/8" particle board.

I'm using 2" staples to fasten it down. Should I still tear out the particle board and replace with OSB?

Thanks, Brian.

Particle board will expand and get spongey if it gets wet. Some people say that fasteners do not hold well in particle board. I find that using coated fasteners hold pretty well in my area. Go for it!