When Sandstone Oracle enters the battlefield, choose an opponent. If that player has more cards in hand than you, draw cards equal to the difference.
You choose an opponent while Sandstone Oracle’s ability is resolving. No player may take actions between the time you make this choice and the time you draw cards.
To draw cards equal to the difference, first determine how many cards you’ll draw, then draw that many cards, as modified by replacement effects. For example, if you have two cards in hand and the chosen opponent has five, Thought Reflection will cause you to draw six cards instead of three.