A market-on-close order seeks the closing auction without a limit price. Understand its unknown cash cost, allocation risk, deadlines and backtest timing.
Market-on-close order
A market-on-close order (MOC) requests execution in a closing auction without specifying a limit price. It lets a portfolio seek the closing benchmark, while leaving the eventual price—and therefore the cash required for a share quantity—unknown when the order is decided.
A MOC instruction belongs to a particular broker, instrument and venue. It is different from sending an ordinary market order just before trading ends: that order can encounter continuous trading rather than the auction. Neither instruction guarantees a price or unconditional completion.
A quantity commitment with an unknown cash bill
Suppose a portfolio submits an accepted MOC buy for 100 shares in a fictional market. It chooses that quantity using information available before the submission deadline. The closing auction later determines a price of USD 102 per share, and we stipulate that all 100 shares receive an allocation. Ignore fees.
Confirmed trade item
Result
Shares acquired
+100 shares
Purchase consideration
USD 10,200
Cash change for the buyer
−USD 10,200
Unfilled quantity
0 shares
The purchase consideration is 100×102=10,200 USD. This is a fill outcome, not a cash budget guaranteed at submission. The buyer did not authorise a maximum price of 102; 102 became known when the auction determined it.
Synthetic MOC buy: an earlier decision commits to buy 100 shares without a limit. The later auction price is USD 102 per share and full allocation is stipulated. The confirmed ledger adds 100 shares, deducts USD 10,200 before fees and leaves zero shares unfilled.
Original synthetic timeline and buyer's trade ledger. Prices are USD per share, fees are excluded and full allocation is an explicit assumption. Event spacing is conceptual, not elapsed time or a venue timetable.
A different allocation changes the ledger even at the same price. If only 60 shares fill at 102, the purchase consideration is USD 6,120 and 40 shares remain unfilled. An order for 100 is an instruction; only confirmed fills create the acquired position. A completely unfilled order has zero purchase consideration, before any separately applicable charges.
Which risk does MOC accept?
A portfolio valued against the closing price may want its trades to share that reference. MOC can reduce the mismatch between its execution reference and that benchmark. It does not remove market impact, fees, uncertain allocation or the possibility that the closing price is unattractive relative to the earlier decision price.
A limit-on-close order adds a maximum buy price or minimum sell price. That bound can prevent execution at an unacceptable price, while leaving more of the requested trade incomplete. The choice is a policy decision about price protection and completion; a favourable-looking closing price does not settle both questions.
SelfMadeQuant's Market, Limit and Conditional Orders follows orders through acknowledgement, partial execution and cancellation, so desired quantities cannot be mistaken for positions. Its preview is public; the full lesson requires membership.
The deadline is part of the instruction
For a normal Nasdaq trading day, the current rulebook, checked 27 October 2026, provides these boundaries in Eastern Time:
Action
Nasdaq MOC rule
Entry
Before 3:55 p.m.; entry at or after 3:55 is rejected
Ordinary cancellation/modification
Before 3:50 p.m.
Legitimate-error correction request
From 3:50 until before 3:58 p.m.
Cancellation/modification from 3:58 p.m.
Not permitted
In the ordinary Closing Cross, MOC orders rank first, with time priority among them. Cross-price benchmark controls still apply. Unexecuted MOC quantity is cancelled under the cross-only lifetime rule; the Extended Trading Close provisions for eligible LOC orders do not make MOC residuals eligible. Early closes, auxiliary procedures and halt crosses require their own rules.
Interactive Brokers' MOC instructions separately publish Island receipt by 15:55 and no cancellation/modification after 15:50. Check the broker's current route-specific deadline and allow delivery time; a client submission timestamp does not establish exchange receipt before its cutoff. Confirm acceptance and subsequent execution or cancellation reports before recording the final order state.
A close in a dataset is not an earlier signal
The final auction price can value a confirmed fill of an already-submitted order. Using that same final price to decide whether to place the earlier MOC order introduces look-ahead bias. An indicative price available before submission is a different observation and can change before the auction completes.
A backtest should retain the information available at the decision, the broker/venue arrival assumptions, and a stated fill and remainder policy. Assigning every desired trade the daily close silently assumes both timely auction participation and full allocation. Those assumptions need evidence or explicit modelling; the closing-price column alone supplies neither.
Rishi K. Narang, Inside the Black Box: A Simple Guide to Quantitative and High-Frequency Trading, 2nd edition, Wiley, 2013, Chapter 7, “Other Order Types”, pp. 124–125: closing-auction order context and venue variation. The ledger and timeline are original synthetic examples.
