We curently have carpeted stairs from the main level to the second level. We are replacing the stairs with hardwoods (oak.) The steps are OBS and so are the risers. We will we tearing out and replacing both treads and risers. We have already bought the treads and have stained them to the desired color. Should we polyurethane them before installation? If we do then when we nail the stairs in place we will have "holes" that will show even if we fill with wood putty. If we wait then it will be difficult to keep the kids off the stairs... I assume we need to do our poly coats first, what is the best way to attach the stairs without visible signs?

yes go and poly them. put on as many coats as you like. the nail holes, you just need to fill them in with the color putty that you have stained them. puting new treads and rises on, do it the way you thing you can do them. there is a couple of ways you can do them. but doing staircase is very easy if you know what you are doing. most of our buisness is in staicases, we build and remake them like the one you have. if you like you can try and call the office and ask for me (Jeff) and i'll try to help you out as much as i can. if you have side skirts with the dado of the tread/risers on the side you can pop the drywood off the back side and slide them in and shim them that away.
Jeff

Hello .
If you have stained the stair threads and risers already then of cours apply the finish, apply two coats , then install the stairs , then find the wood filler that's the same color as the stair thread, fill the nail holes with the filler sand the floor with 180 grit paper and apply the final finish coat.