All-or-None (AON)

All-or-None (AON) is an instruction you attach to an order telling your broker: fill my entire order in one go, or don't fill it at all. What you don't want is a partial fill — say, getting 200 shares of a 1,000-share order because that's all the liquidity available at your price at that moment, leaving you with an odd, unintended position.

It works as a condition layered on top of a regular limit (or sometimes market) order. The order sits and waits. If the full quantity you asked for becomes available at your specified price, the whole thing executes at once. If only part of it can be filled, the order simply does not execute at all, and it keeps waiting for a moment when the full size is available.

The nuance that trips people up is the difference between AON and Fill-or-Kill (FOC, sometimes called Immediate-or-Cancel when partial fills are allowed). Both share the "all or nothing" quantity requirement, but they differ in patience: FOC demands that fill happen right now or the order is cancelled immediately, while AON is willing to wait — typically until the end of the trading day, or until you cancel it yourself. So AON trades urgency for completeness: you might wait hours for a fill, or get none at all if the size never appears.

Because AON orders are conditional and can sit unfilled, many brokers and exchanges route them differently than plain limit orders, and not all trading venues support them for all order types. This is a mechanical detail — how your specific broker or exchange handles AON routing — that varies and is worth checking rather than assuming.