Orders, Volume, and Open Interest

An option order specifies a proposed transaction’s contract, quantity, and execution terms. An execution, or fill, completes some or all of that requested transaction. Completed trades contribute to trading volume, while their opening or closing designations help determine how many contracts remain outstanding as open interest. These measures describe different stages of market activity and should not be treated as interchangeable.

Opening and closing instructions

Buying to open establishes a long option, and selling to open establishes a short option. Selling to close reduces a purchased position; buying to close reduces an open short position. A closing trade must match the series being offset, including the underlying, option type, strike, expiration, and deliverable. Purchasing protection at another strike creates a separate leg rather than closing the original obligation.

A fill can be partial. If an order to close five contracts executes for only two, three contracts remain open. The remaining order quantity may continue to work according to its instructions until it fills, expires, or is successfully cancelled. Submitting a closing order by itself does not remove the unfilled position’s exposure.

Market and limit orders

A market order seeks execution at the best available prices when it reaches the market, subject to applicable execution controls. Purchases trade against offers and sales against bids. The quantity available at each price determines whether the order can execute at one level or must reach additional levels.

For example, consider a purchase of five SPY $775 calls with 45 days remaining. Three contracts are offered at $8.70 per share and the next two at $9.00. Executing against both offers gives the following premium cost for standard 100-share contracts:

The weighted-average execution price is $8.82 per share. The first three contracts use the initial offer, while the remaining two require a higher price. A quoted ask is therefore not necessarily the average price for the entire intended quantity.

A limit order specifies a maximum purchase price or minimum sale price. An $8.70 purchase limit in the same market could execute the first three contracts while leaving two unfilled until offers become available at $8.70 or lower. A limit inside the bid-ask spread seeks a better price but may wait without executing. Available size and exchange priority determine whether and when competing orders receive fills.

Multi-leg and complex orders

A complex order submits several option legs as a package in a specified ratio. A net debit limit restricts the amount paid for the package, and a net credit limit sets the minimum amount received. The individual leg prices can vary while the combined execution respects the package limit. Cboe’s complex-order mechanisms provide for package trading and price competition at a net price.

Buying one $750 call and selling one $775 call on the same underlying, with the same expiration and contract size, creates a one-to-one call spread. A $10 net debit limit permits a payment of no more than $1,000 per standard spread before fees. The limit applies to the difference between the two premiums rather than specifying a separate maximum or minimum for each leg.

Electronic complex-order fills maintain the specified ratio for the portion executed. An order for five one-to-one spreads can fill two complete spreads and leave three packages unfilled. The filled portion contains two contracts in each leg; the unfilled portion remains subject to the order’s instructions.

Executing the legs separately creates legging risk. A price change between fills can alter the intended net cost, and the interim position can have substantially different risk from the completed spread. Package execution addresses that sequencing problem for the portion filled, but it does not guarantee that the whole order will execute or remove subsequent exercise and assignment risk.

Execution prices and valuation marks

A displayed position value may use a mark based on the midpoint, last trade, or a theoretical calculation. A closing transaction instead realizes the execution prices available for its size. A long option marked at the midpoint may sell nearer the bid; a short option may cost nearer the ask to repurchase.

Wide spreads and limited depth can make the difference between marked and realized profit or loss substantial. Execution inside the spread can improve the result relative to immediately trading at the displayed bid or ask. Fees further reduce net profit or increase net loss. The accounting reference price and the actual transaction price therefore need separate treatment.

Trading volume

Volume is the number of contracts traded during a session. A five-contract transaction adds five to the count, and the measure begins again for the next session. Opening trades, closing trades, and hedging activity all contribute. The same economic exposure can also pass through several holders within the day.

Volume is specific to the series being measured. Heavy activity at one strike or expiration does not establish comparable activity in another series on the same underlying. Nor does session volume describe the quantity currently offered at a particular price.

Open interest

Open interest counts contracts that remain outstanding after clearing. Each contract is counted once even though it has both a long and a short side. A transaction creates new open interest when both sides open, removes it when both sides close, and transfers an existing position when one side opens while the other closes.

Suppose a series begins the day with open interest of 1,000. Transactions that open 100 new contracts, close 40, and transfer 200 existing contracts generate volume of 340. The transfers contribute to volume but do not change the outstanding total. With no exercises or other adjustments, closing open interest is:

Exercise and expiration also remove outstanding contracts. Published open interest incorporates the relevant clearing activity and is generally updated daily, while volume accumulates throughout the session. A high volume figure therefore need not produce a similarly large increase in open interest.