I bought a fair amount of 3/4" red oak last week and when I got home and started tearing the tile and carpet out, I quickly discovered I had 1/2" particle board underlayment over 1/2" plywood subfloor. On an earlier post of mine, I got recommendations and it seems like 1/2" plywood or osb would work but most seem to prefer the plywood. I was going to go with osb because I thought it was quite a bit cheaper than the plywood and the discovery of particle board leads to another unexpected expense. Tonight, at the store I was checking prices and it seems like I can get 1/2" plywood for only about 2 bucks more a sheet than osb but I think they called it black line. Green line was even more yet. My question it, does it matter what grade I use or should I just stick with osb. I was not expecting to find the particle board, so I'm trying to keep costs down as much as possible however, I do want to make sure the job is done correctly due to the time and expenses I will have in it. Thanks in advance for you help.

More than likely what you have now underneath the particle board is (black line) CDX sheathing grade plywood. Not real good because there are voids within the inner plies and it lacks a T&G. My personal preference is to use an underlayment grade plywood (no inner voids) to replace the particle board. That may be the green line you mentioned, but ask for AC(X) or BC(X) grade.... the BC pine will be cheaper than the AC fir.

Thanks. Do you see a huge problem with going with 1/2" osb? Would the combination of that and the existing plywood subfloor provide enough "hold" for the nails/staples?

A HUGE problem? No. Is plywood better than OSB? Most/many wood floor installers think so. That said, OSB is aNOFMA/NWFA approved subfloor material. I'd suggest using a flooring stapler if using OSB.

Sounds like I'll be better off to spring for the plywood. Thanks for your help. I look forward to getting this out of the way so I can start laying the hardwood! Then it will all be worth it.

Thanks again everyone for the help. I got all of the carpet, padding and particle board removed from the living room, hallway, entry way and closet and replaced it all with 1/2" bcx. It did cost quite a bit more than osb or black line but I only want to have to do this once. I was able to get rid of all of the squeeks other than one area. We shot screws everywhere we could and couldn't get them to go away so I might have to put a small jack under it in the basement. Hopefully the new hardwood will help a little too. I will be picking up my nail gun this week and starting to lay planks by Friday or Saturday. Can't wait to see how it turns out!