I have a 5/8" particle board over a 1/2" cdx subfloor. Should i remove pb and if so what should i use as a underlayment for my solid hardwood red oak floor i want to install.

YES!!!!!!!!!!!!! Get that particle board out of there.

What's the joist spacing? You're probably going to want to add a minimum 1/2" ply or OSB.

Yes, the particle board needs to be removed and new PLYWOOD underlayment installed. 1/2" plywood is usually sufficient unless the joist spacing is over 20" OC.

I had a customer remove the particle board and have me lay the 3/4 red oak directly on the 1/2. He insisted that it would still be stronger than the junk particle board.

Welcome Jim, your client is likely to regret that a few years down the road. It's commonly done in VA too, but that type of install can end up getting loose and squeaky. The 1/2" CDX sheathing grade plywood is not intended to be used as a single layered subfloor.

We were involved in a gov't housing project a few years back, where they threw out the rule book for construction. Among many other things, 1/2 inch plywood subfloor 24 inch joist spacing and we laid 3/8th inch unfinished oak strip over this mess. What a riot trying to sand it.
The whole project was built to less than minimum standard and the cost to bring it up to reasonable standards later was phenomenal.

thanks jerry. I didn't recomend this method, but the customer insisted.

Unbelievable! 1/2" ply spanned 24" OC ? I didn't even build my tree houses as a kid that bad!