I am planning to install 3/4 inch solid jatoba brazillian cherry throughout my house and on the stairs which have existing carpet. First question is
can I use the existing stair material for a riser. I.E. The stairs are built with the OSB board, and probaly would not look good painted. So would I remove the OSB board and then put a solid riser board there, or just put a solid riser board over the existing board? Next question, is around the top of the stairs, there is an existing trim piece where my spindles tie into. This piece would not be level when I put my new hardwood in. Should I take this piece up, and put down a stair nose piece?

Thanks in advance for the help.

I do not remove the existing treads and risers when overlaying with hardwood flooring. I trim the extended nosing off of the rough tread and overlay both tread and riser. For the overlay riser material, you can use the flooring, a hardwood plywood that matches or paint grade wood if you plan on having painted risers. As far as the existing wood piece that the balusters are installed into, sometimes those can be made to work with a little ingenuity. Personally, I try not to dismantle handrails systems to install flooring unless there is no other way. Oftentimes, that existing baluster "plate" can be made to work with a molding or by butting to it. Read this:
http://www.hardwoodinstaller.com/hardwo ... stairs.htm