What color of stain should I use on my hardwood floor?

First, you'll to need to have the flooring sanded to remove the oil based stain on it. Stain soaks into the grain of wood, so a light sanding won't remove it. I'd explain the problem to someone at a paint store that also sells wood stains. The clerks will be more knowledgeable than at a big box store. Then I'd follow their expert advice.

Are you really going to sand and restain all your floors just so you can MAYBE pull up the floor half a shade? Work and money wasted, is all I can say. Oak used for flooring is naturally red or golden (unless you know for sure it is bleached, white oak, but then it wouldn't have taken on a golden hue with the oil-based natural stain), and there is nothing going to lighten it barring painting it (which, of course, destroys the value of your floors and ergo your home), you can only darken it with stain. Better to get some light colored rugs and hang mirrors to reflect and enhance what light you have.

You can still have a dark colored floor and a light room, as long as your paint the walls a light color, use extra lighting and mirrors, the color of the floor isn't going to make that much of a difference.

If I was in your shoes and I didn't like naturally finished oak, I would consider one of the following; 5% white Rubio Monocoat (very subtle white), Duraseal Special Walnut (for a medium brown shade), Duraseal Classic Grey. Hope this helps.