Post-Only orders get canceled for a simple reason: the price you set could be immediately filled at that moment. If that happens, your order turns into a taker order that eats into the order book, and the system will automatically cancel it. This rule is strict — no workarounds.
Why a price that matches the market gets canceled
To understand this, you need to know the two roles in trading:
Maker: Your order sits on the order book without being filled right away. You provide liquidity. Maker orders usually get lower fees.
Taker: Your order matches an existing order on the book immediately. You "take" liquidity. Taker orders pay higher fees.
A "Post-Only" order forces your order to be a maker only. When you place it, the system checks: if this order would instantly match an existing order on the book, it means you are acting as a taker. The system then cancels it straight away — it never gets the chance to fill.
Example: how your order gets canceled
Imagine the lowest sell price on the BTC/USDT order book is 18,737.25 USDT:
Order succeeds (Maker): You place a buy order at 18,726 USDT, below the current market price. This order goes onto the order book and waits. You act as a maker providing liquidity, so the system keeps it.
Order gets canceled (Taker): You place a buy order at 18,737.25 USDT or higher (e.g., 18,745 USDT). This price equals or exceeds the lowest sell price. The moment it reaches the book, it matches an existing sell order instantly. You are "eating" the sell side — that's taker behavior. The system immediately cancels your post-only order, because its whole purpose is to stop you from being a taker.
Other common reasons for failure
Besides the most common reason above, your post-only order may also be canceled because of:
Self-trade prevention (STP): To keep trading fair, OKX does not allow different sub-accounts under the same main account to trade with each other. If your order could match another order from the same main account, the system will cancel the maker order.
Account or product-specific limits: In some special cases — for example, certain token contract liquidity enhancement plans (ELPs) that only allow makers, or risk control measures that temporarily block taker actions for some accounts — these limits can also cause a post-only order to be canceled.
Risk note: If your strategy is to save on fees with maker rates, but you also worry about missing a chance when the price moves, consider placing your post-only order at a price slightly below the current market price, and watch the order book depth closely. Before placing an order, check the nearest best bid/ask on the order book, then decide your limit price.
After the operation
How to verify: Look in your order history or current pending orders. If you see status "Canceled" or "Cancelled", check whether it was due to the post-only rule or STP logic.
Next step: If getting filled is your biggest priority, try an Immediate-or-Cancel (IOC) order instead. It allows partial fills and cancels the rest, so your order won't be thrown away just because it can't fill completely in one go.
