When Overzealous Fan triggers, can I react with Ambush?

Yes, you can react to Overzealous Fan with a unit that has Ambush.

According to the RiftJudge FAQ #10160, #10158, and #9611, the interaction functions as follows:

The Timing Sequence

1. Trigger Condition Met: When Overzealous Fan gains the "Defender" designation during a combat showdown, its ability triggers.
2. The "May" Decision (Upfront): Because the ability text says "you may," the controller of Overzealous Fan must decide immediately whether to use the ability.
   • If they choose to use it, they must kill the Overzealous Fan immediately as a cost to place the ability on the chain.
   • If they choose not to use it, the ability does not go on the chain at all.
3. Reaction Window: Once the ability is placed on the chain, the state becomes CLOSED. You may then play your unit with Ambush as a [Reaction].

Key Takeaways

• Costs are Upfront: The controller of Overzealous Fan cannot wait to see if you play an Ambush unit before deciding to kill the Fan. They must commit to the cost of killing the Fan (to place the ability on the chain) the moment the trigger condition is met.
• Ambush Timing: Once the ability is on the chain, the game state allows for reactions, which is when your Ambush unit can be played.

This ruling is based on the updated Unleashed rules, which clarify that "kill me" instructions in "you may" triggered abilities are costs that must be paid upfront to place the ability on the chain (RiftJudge FAQ #8702).