My oven has only one shelf. Is it ok to put a cooling rack on the bottom of the oven to make another shelf?

I would avoid mcgyver-ing in this situation. Dropped pans can result in burns, fires, other Thanksgiving disasters. Ideas for you:
* another real oven rack,
*use stovetop, grill, mw for heating.
*make some dishes that can be served at room temp.

If it's oven-safe and sturdy I would say it's worth a try, but my biggest concern would be that cooling racks aren't terribly weight bearing. If there's time to run out to a hardware store / kitchen store to get something sturdier and more fitted, that's the way I would go. Or, make a bunch of dishes ahead so you don't have to deal with the oven jockeying!