You are unfortunately running into a major defect of OGS’s undo feature, namely you can only ask for undo on your opponent’s move. So if they reply fast, you can’t ask for undo. It would be better if you could also ask undo on your move, thus undoing your move and the opponent’s reply. This is how undo works on every other server I have used. Here’s my feature request for it here: Allow undo request on your move
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