I am planning to install an 3/4 prefinished oak floor in a new construction house that is being built for me.The subfloor is 3/4 playwood.I have a 16x24 room and the joists run the length of the room Do I have to cross the joists or can I run the planks parallel with them

Most manufacturers and NOFMA state the flooring should run at 90 degrees, or perpendicular, to the floor joists. If you wish to run parallel, you can either add another layer of 1/2" plywood OR add blocking 16" OC underneath the floor between the floor joists. You could also run the flooring at a 45 degree angle as well.