3/4" Thick Solid Wood Flooring can be installed directly over 3/4" thick OSB subfloor. It is standard practice to install the hardwood floors diagonally across the floor joists to stabilize and strengthen the whole floor structure by cross layering the wood flooring to the floor joists and the sub flooring. If the existing subfloor is 3/4" thick, solid and flat with no deflection when walked on you, can run the wood flooring parallel with the floor joists to obtain a desired look. If the OSB sub flooring moves when walked on then you should add an additional layer of 3/8" or 1/2" thick plywood underlayment glued and screwed down to add stability. OR install 2x6 blocking between the floor joists for added stability. Running the floorboards along the longest dimension of the room will make the room appear larger than it is. We recommend laying 15 lb. black (roofers) felt paper over the subfloor to help retard moisture transfer.