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In many Uno Online modes, if someone wins, the losers are penalized based on the cards left in their hands. Numbers ($0-9$) are worth their face value, but Action cards ($+2$, Skip, Reverse) are 20 points, and Wilds are a massive 50 points. Get rid of high-value cards early to minimize the damage if you don't win.

In many online versions, you can "bluff" a $+4$ even if you have a matching color. Use this when your opponent is down to their last 1-2 cards. Watching someone draw 4 cards right at the finish line is... evil, but effective.

Most online platforms (like Uno on Facebook or Mobile) allow Stacking. If someone hits you with a $+2$, you can play your own $+2$ to pass a $+4$ to the next person. Learn the house rules of the platform you're on to create massive "draw combos" that can force a player to pick up 10+ cards at once.