Why did my Post only order disappear the moment I placed it?
Why a limit buy "parked at the market price" filled instantly
The best ask on the BTC order book is 64,000.1 USDT (an illustrative price). You want to buy a little but you're in no hurry, so you place a limit buy at 64,000.1, thinking you'll leave it there to wait and pay the maker fee while you're at it.
You submit it and it fills in full straight away. Check the trade history and you were charged the taker fee.
That's because 64,000.1 is exactly the best ask: someone is already waiting to sell at that price, so your buy order matched against it the moment it arrived. A limit order only guarantees a price no worse than the one you set; it doesn't care whether you end up as the maker or the taker in the fill. For what each basic order type does and doesn't guarantee, there's a quick-reference table in order types to know before your first OKX trade.
The second time, you try something else: under the advanced limit order you choose Post only. Same price, 64,000.1. This time the order is gone the moment you submit it: no fill, and nothing resting on the book either.
Two opposite outcomes, one underlying question: at the moment you submit, can this order match against an order already on the book? A regular limit order that can match simply fills; a Post only order that can match is simply cancelled.
In short: a Post only order is guaranteed to go onto the order book, which makes you a market maker, and if it could match an existing order immediately, it is cancelled.
How much does a maker fee save? 20 trades a month, worked out
What Post only can save you is the gap between the maker and taker fee. According to OKX's trading fee page (checked October 2026), regular users pay 0.08% as a spot maker and 0.1% as a spot taker. The same page defines both terms: a maker order goes onto the order book without filling immediately, and a taker order matches an existing order on the book and fills immediately.
At regular-user rates, 20 trades a month at 3,000 USDT each cost 60 USDT in fees if every trade is a taker trade, and 48 USDT if every trade is a maker trade.
On a single trade, the difference hardly registers. It matters through repetition: the gap applies to every trade, so the more you trade, the more it adds up to money you can actually see.
There is a catch: the fee gap only pays off when the order actually fills. If a Post only order is cancelled, or sits on the book with nobody taking it, you save nothing on fees and you don't get what you wanted to buy either. You also can't know in advance when, or whether, it will fill.
What gets a Post only order cancelled
There is one cancellation condition: the order could match an existing order immediately. Applied to actual prices, that gives two rules:
- Buy orders: if your price is greater than or equal to the lowest ask on the book at that moment, the order can match immediately, and Post only cancels it.
- Sell orders: if your price is less than or equal to the highest bid on the book at that moment, the same thing happens.
With the best ask at 64,000.1, a Post only buy at 64,000.1 meets the matching condition and is cancelled. Change it to 64,000.0, below the best ask, and nothing on the other side can fill it immediately, so it goes onto the book and waits.
In practice, these are the usual ways traders run into the cancellation condition:
- The book moved around the time you submitted. When you looked, the best ask was 64,000.2, so 64,000.1 seemed to leave one tick of room; by the time the order was processed, the best ask had already dropped to 64,000.1 or lower. In a fast market that's plenty of time for the book to shift a tick.
- You wanted a quicker fill, so you entered the best price on the other side. For a regular limit order, that's an order that fills at once. For Post only, it's exactly the cancellation condition.
- You picked the wrong side. You meant to place a sell order and wait for the price to rise, chose Buy instead and set the price above the market, so it ran straight into the asks.
Being cancelled just means the order didn't fill and didn't stay on the book. Whether to place it again, and at what price, is up to you.
If your Post only orders keep getting cancelled, your price is usually too close to the other side. Move back a tick or two toward your own side and the cancellations will drop off, though your orders will fill more slowly.
Post only vs FOK vs IOC: one table
OKX puts these three under the advanced limit order, next to the regular limit order. A regular limit order, once it's on the book, stays there until it fills or you cancel it. The three advanced options all change what happens at the moment you submit.
| Option | Goes onto the order book? | What happens when you submit | In one line |
|---|---|---|---|
| Post only | Yes, and it only ever exists as an order resting on the book | Its rule works the other way round: what it guards against is filling immediately. Can match immediately → the whole order is cancelled; can't match immediately → it goes onto the book to wait for a counterparty | Guaranteed to go onto the order book and make you a market maker; cancelled if it could match an existing order immediately |
| Fill or kill (FOK) | Never stays on the book | Can fill in full immediately → the whole order fills; can't fill all of it → the whole order is cancelled and nothing fills | Must fill in full immediately, or the entire order is cancelled |
| Immediate or cancel (IOC) | Never stays on the book | Whatever can fill immediately fills; the rest is cancelled | Must fill immediately; any part of the order that doesn't fill is cancelled |
FOK and IOC both require an immediate fill, and anything that doesn't fill immediately is cancelled. There's no resting-and-waiting stage, which is why the table says they never stay on the book.
Put the three rows side by side and the split is clear: Post only wants not to fill immediately, while FOK and IOC want to fill immediately. FOK also insists on the full quantity; IOC accepts a partial fill.
An illustrative case to tell the last two apart: you want to buy 1 BTC, the best ask only has 0.4 BTC on offer, and the next price level up is already above your limit.
- FOK: 1 BTC can't be filled in full, so the whole order is cancelled and you buy nothing.
- IOC: you buy 0.4 BTC, and the remaining 0.6 BTC is cancelled.
- Regular limit order: you buy 0.4 BTC, and the remaining 0.6 BTC stays on the book at your limit price and keeps waiting.
- Post only: your limit price can match the best ask, so the whole order is cancelled and you buy nothing.
When to use Post only, and when to switch it off
When it's worth ticking:
- Building a position or buying in batches when you're not in a hurry. If you were already planning to sit below the best ask and wait, ticking Post only costs you almost nothing, and it stops a mistyped price from turning the order into a taker trade.
- You trade often. As worked out above, the gap adds up trade by trade, so the more you trade, the more this option is worth making a habit.
- You've set yourself a rule to fill only at maker rates. Post only hands that rule to the matching engine, so you don't have to check the book before every order to make sure your price hasn't crossed the other side.
When to untick it:
- When you need out now. Stop-losses, trimming a position, getting out after a sudden reversal: what you need then is a fill, and Post only's cancellation condition kicks in exactly when you most want one.
- When the order book is moving fast. With the price pushing toward you tick by tick, an order priced close to the other side is likely to be cancelled, and one priced further away won't catch up. Placing it again and again is more hassle than the extra taker fee.
- When amounts are small and trades are few. The gap is a percentage: on a 100 USDT order at regular-user rates, maker and taker fees differ by just 0.02 USDT (100 × 0.02%).
FOK and IOC sit at the other end. When you need to fill right now but won't accept a price beyond your limit, that's their job: if the full quantity has to fill in one go or not at all, choose FOK; if you'll take whatever fills and don't want the rest left on the book, choose IOC.
The first time you use any of these options, place a small order and check that the result matches what you expected before you use it on a meaningful amount.
Public sources
- OKX trading fee page (checked October 2026). Source for the regular-user spot maker and taker rates, and for the definitions of maker and taker.
FAQ
My buy price was below the last traded price. Why was my Post only order still cancelled?
The cancellation check looks at the opposite side of the order book, not at the last traded price. Right after a drop, the last price might still read 1,005 while the best ask has already moved down to 1,000. Put in a buy at 1,002 and it is below the last price but above the best ask, so it can match immediately and gets cancelled all the same. Before you set a price, look at the best ask (or the best bid if you are selling), not at the last price that keeps flickering.
If my Post only order rests on the book and then fills, is that a maker or a taker trade?
A Post only order either goes onto the book or gets cancelled, so the only way it can fill is when someone else's order comes in and matches it. OKX defines a maker order as one that enters the order book without filling immediately, so the trade counts as maker.
Are FOK and IOC orders charged maker or taker fees?
Both fill at the moment you submit, by matching orders already sitting on the book. Going by OKX's definition of a taker, those fills are taker trades. These two options won't save you fees; what they control is how much fills and when.
Can one order be both Post only and FOK?
OKX lists Post only, FOK and IOC as three options of the advanced limit order. Going by the rules alone, Post only requires that the order cannot fill immediately and FOK requires that it fills in full immediately; put both conditions on one order and no outcome can satisfy them. Pick one of the three based on what worries you most about that particular order.