Iceberg, Post-Only and TWAP: Execution Orders Beyond Market and Limit
Market and limit orders answer two questions: at what price, and how much. A second family of orders answers different ones: whether the order may take liquidity, whether its full size should be visible, and whether it should be worked over time. This guide reads the documented behavior of post-only, iceberg and TWAP orders, the parameters that define them, and the pending-order limits that apply.
Cover artwork: CoinSoso · AI-generated, not a photograph of a real venue, product or market capture.
Key takeaways
- Post-only is a constraint on how an order may fill: Bybit documents that an order which would fill immediately is cancelled instead.
- An iceberg order hides total size by splitting it into smaller limit orders; OKX exposes szLimit, lmtOrderNumber, aggressiveness and pxLimit for its smart_iceberg type.
- A TWAP order splits size across regular time intervals; OKX documents timeInterval in seconds and szLimit as the average amount per slice.
- OKX caps pending algo orders at 100 iceberg orders and 20 TWAP orders, alongside 500 trigger orders and 50 trailing orders.
- None of these order types guarantees a fill; they change how an order is worked, not whether the market trades.
The questions market and limit do not ask
A market order says “fill now at whatever the book offers”. A limit order says “fill at this price or better”. Both are answers to the price question. Neither says whether the order is allowed to take liquidity, whether its size should be displayed, or whether it should be worked over an afternoon rather than submitted at once.
The order types in this guide answer those questions. They are not a separate market; they are instructions for how an order should be handled once it reaches the venue.
Post-only: a constraint on taking
Post-only is the simplest of the three. It attaches a condition to an order: the order may rest in the book, but it may not fill immediately on arrival. Bybit’s API documentation states the rule directly: if a post-only order would be filled immediately when submitted, it will be cancelled.
That behavior is the point. A trader who wants to be a maker — to earn the maker side of the fee schedule and to avoid paying the spread — uses post-only to ensure the order does not accidentally cross. The cost is that the order may be cancelled instead of filled, which is a different failure from a limit order that rests unfilled.
OKX exposes post-only as its own ordType value, post_only, alongside market, limit, fok, ioc, optimal_limit_ioc and the market-maker protection variants. It also has an instrument state called post_only in which only post-only orders are accepted and other order types are rejected. The two uses of the term are related but distinct: one is an order type, the other is a venue-level restriction on an instrument.
Iceberg: splitting size
An iceberg order hides the full size behind a smaller visible quantity. OKX’s smart_iceberg type documents the controls:
| Parameter | What it controls |
|---|---|
sz |
Total order size |
szLimit |
Minimum order size per execution |
lmtOrderNumber |
Number of limit order splits |
aggressiveness |
radical, mid or conservative |
pxLimit |
Price limit for the order |
triggerParams |
Optional trigger conditions before the iceberg starts |
The design is a sequence of smaller limit orders rather than one large one. lmtOrderNumber sets how many splits the total is divided into, and szLimit sets the floor for each execution. aggressiveness describes how the slices are placed: OKX labels conservative as queueing at the best bid or ask, mid as faster fill with a better price, and radical as faster fill.
What the mechanism does not do is hide the executions. Each slice that trades is visible on the tape, and a repeated pattern of small orders at the same price can be read as a larger intention. The iceberg reduces the signal in the visible book; it does not remove the signal from the record.
TWAP: splitting time
A time-weighted average price order divides the total into slices and submits them at regular intervals. OKX describes the strategy as splitting an order and placing smaller orders at regular time intervals, and lists the parameters that define the schedule:
| Parameter | What it controls |
|---|---|
sz |
Total order size |
szLimit |
Average amount per slice |
timeInterval |
Time between slices, in seconds |
pxLimit |
Price limit |
pxVar |
Price variance as a percentage, range 0.0001 to 0.01 |
pxSpread |
Price variance as a constant |
The two variance parameters are alternatives: OKX says either pxVar or pxSpread is allowed, and describes them as the range within which small orders are placed relative to the best bid or ask. That is the part that makes a TWAP more than a timer. The schedule says when to act; the variance says how far from the touch the slice may be placed.
A TWAP is therefore a compromise between urgency and impact. Slicing over time reduces the chance that one order sweeps the book, but it also exposes the order to price moves during the schedule. The price limit is the boundary; if the market leaves the range, the strategy has less room to work.
Documented limits
Both venues publish caps that apply before any strategy logic runs. OKX’s order rules list a maximum of 4,000 pending orders across the exchange and 500 per trading symbol. Its pending algo-order limits are separate:
| Algo type | Pending limit |
|---|---|
| TP/SL | 100 per instrument |
| Trigger | 500 |
| Trailing stop | 50 |
| Iceberg | 100 |
| TWAP | 20 |
The TWAP cap is the tightest of the group. A desk running many small schedules can reach twenty long before it reaches the iceberg or trigger limits, which makes the count of concurrent TWAPs an operational constraint rather than a detail.
Choosing between them
| If the goal is… | The order type that addresses it | The trade-off |
|---|---|---|
| Avoid taking liquidity | Post-only | The order may be cancelled rather than filled |
| Reduce visible size | Iceberg | Executions still print; the hidden size is inferable |
| Work an order over time | TWAP | Exposure to price moves during the schedule |
| React to a price level | Trigger or stop order | The trigger is not a guaranteed fill price |
The table is a starting point, not a rule. A post-only order inside a spread that never trades is not a fill; an iceberg in a thin book is still a large order relative to the liquidity; a TWAP in a fast market is a schedule that may finish away from its target. Each type changes how the order is worked, and none changes the underlying fact that a fill requires a counterparty.
The common failure
The common failure is treating an execution order as a guarantee. Post-only can cancel, iceberg slices can rest unfilled, and a TWAP can finish with a worse average than the arrival price. What these orders buy is control over how the order interacts with the book — not control over the book itself.
A practical check before using one is to ask which of the three questions the order is answering. If the answer is “I do not want to take liquidity”, post-only is the tool. If it is “I do not want my full size visible”, iceberg is the tool. If it is “I want to spread my participation over time”, TWAP is the tool. If the answer is “I want a better price”, none of them is the tool; that is a limit order and a wait.
Risk note. The order types, parameters and limits in this article were read from OKX’s APIv5 documentation and Bybit’s V5 create-order documentation on 2026-10-26. Parameters and caps change; check the venue’s current documentation before relying on a number. Nothing here is investment advice.
Sources
How this was researched. Written from primary documentation and the exchange or protocol's own published material, with sources listed at the end of the article.
Primary sources for the factual claims on this page. A date appears only where a human has actually read the source; where it is missing, the check is still outstanding.
- OKX APIv5 — Place order, ordType and post-only instrument state(read 2026-10-26)
- OKX APIv5 — Place algo order, iceberg and TWAP parameters(read 2026-10-26)
- Bybit V5 — Create order, timeInForce and PostOnly(read 2026-10-26)
Frequently asked questions
What does post-only actually prevent?
It prevents the order from taking liquidity. Bybit's documentation states that if a post-only order would be filled immediately when submitted, it will be cancelled. The order is allowed to rest in the book; it is not allowed to cross and fill on arrival.
Is an iceberg order the same as a TWAP order?
No. An iceberg splits size into smaller orders without a time schedule as its defining feature; OKX documents a number of limit order splits and a minimum size per execution. A TWAP splits size across regular time intervals, which OKX measures with timeInterval in seconds.
Do iceberg orders hide the total size completely?
They hide it from the visible order book, not from the market. The executions still print, and the repeated small orders can be inferred from the tape. The mechanism reduces the visible signal; it does not make the activity invisible.
Why would a TWAP order miss its target?
Because a TWAP works a schedule, not a price guarantee. OKX's TWAP parameters include a price limit and a price variance, so slices can be constrained by the market. If the price moves outside the configured range, the strategy has less room to execute.
Are algo orders counted against the same limits as normal orders?
OKX publishes separate caps. Its general pending-order limit is 4,000 across the exchange and 500 per symbol, while its pending algo-order limits include 100 iceberg orders, 20 TWAP orders, 500 trigger orders, 50 trailing orders and 100 TP/SL orders per instrument.
Where to go next
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