I feel partially responsible since I steered you in the direction of stripping the wood.
I think you would have noticed those depressions if you were gouging the wood with the putty knife. Oak's a hard wood and you would have felt them.
What grit paper did you start with? If you used anything below 100, that is too coarse. If this would had been finished before, you should be using 120 to start with and moving up in numbers (the lower, the coarser the grit). I would try and see if you could sand out with 100, then 120, then maybe 150.
Maybe practice on another piece of scrap to get the hang of sanding out the wood.