Fill-or-Kill (FOK)

What Is a Fill-or-Kill Order?

A fill-or-kill (FOK) order must be completed in full, immediately, or not at all. When a FOK order reaches the market, the broker checks if the entire quantity can be filled at the specified price right now. If yes, the trade executes. If no, the entire order is cancelled — no partial fill, no waiting.

FOK combines two hard requirements: completeness (entire order must fill) and immediacy (must fill now). Any deviation results in cancellation.

Why It Matters for Options Traders

FOK orders are used by institutions executing large orders where partial fills create more problems than no fill. If a fund requests 2,000 contracts and receives only 400, they now hold an unintended partial position — exposed without their intended hedge ratio, still needing to fill 1,600 contracts at potentially worse prices.

The all-or-nothing logic eliminates this. FOK either achieves the full position or leaves existing exposure unchanged, allowing reassessment and another attempt.

For options, FOK is common when:

• Executing large spread orders where leg-ratio integrity matters
• Entering positions ahead of catalysts (earnings, macro events) where timing is critical
• Avoiding signaling intent by preventing small partial fills from appearing on the tape

The tradeoff is rejection risk. Illiquid options with wide spreads and thin books often cannot absorb large FOK orders, resulting in repeated cancellations.

Key Characteristics

• All-or-nothing: Entire order fills or none of it does — no intermediate outcomes
• Immediate requirement: Live for milliseconds at most, unlike limit orders that rest in the book
• No partial fills: Distinguishes FOK from immediate-or-cancel, which allows partial fills
• Liquidity demand: Large FOK orders require concentrated liquidity at a single price level
• Auto-cancellation on failure: If full quantity cannot fill immediately, order is cancelled
• Common in spreads: Multi-leg orders often use FOK to ensure both legs execute together