Sanding a Home Depot Stair Tread
Yes it is something you can sand out. Be careful you only have about an 1/8" of material to work with and with aggressive sanding that can "burn through" quickly.

You will need to sand all the imperfections out with the 80G move up to 100 or 120G when you have the surface to your satifaction with the 80G. After all the sanding marks from the 80 is gone, change the grit again to 150g and remove all the marks left by the previous grit. You can check your progress of removing the scratches from the previous cut by marking the surface with a pencil and sand until the pencil goes away. I use a large "S" pattern to cover the area with marks about 2" apart, you could do the same with straight lines too. Some people say the graphite is "chased into the grain" and makes more sanding happen than needed. I have not had that problem with oak, pine yes. You could wet the surface you sanded with a fast drying compound that acts like a temporary finish so the scratches show up for a short while. Denatured alcohol or lacquer thinner comes to mind here, but then you have fumes and getting blowed up problems to contend with.

One last important note about oak. It's hard and softwood sand differently, and because of that nature, it will create an irregular surface to the perfectionist eye. The soft pad of a random orbit sander exaggerates this nature. IMHO, it is a little late to flatten it out, the sanding would have been better with a sanding block by hand and not machine. Machines have no discretion as to how much material it removes and where it removes it.