Black stains on oak can be caused by iron in water that gets on the wood reacting with tannens in the wood. I would start by applying oxalic acid to see if that removes the stain. If it does, then strip off the old finish and re-do.
Try putting some lacquer thinner on the spot and allow it to dry. Don't touch it, just wet it and let it sit. If it looks good while wet then it is salvageable. Otherwise the water damage has gone too far and would need to be refinished. Refinishing is difficult for the DIY do to government interference with the chemical companies. It's not a winter sport anyway, even professional grade removers don't do well at all below 70 degrees.
If the spot goes away wet with lacquer thinner then spray the top with lacquer sanding sealer sanding between coat until you level the finish and then topcoat with a couple coats of lacquer.
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