What do buy to open and sell to close mean on an options trade?

Buy to open means a trade that starts a new options position, or adds to one, by buying contracts. Sell to close means a trade that reduces or ends a position someone already holds by selling those contracts. Every options trade has two parts to its name: the side (buy or sell) and whether it opens or closes a position. Put together, that gives four labels: BTO, STO, BTC and STC.

This post is part of our lingo series. It explains the four labels, why the opening or closing side matters when you read the options tape, and what a label can and cannot tell you, using one real print from the tape.

What the four labels mean

An options position is opened and later closed. The definition used by most brokers and references is simple: an opening transaction creates or increases a position, and a closing transaction reduces or ends it.¹ Our opening and closing glossary entry covers the same idea in one paragraph.

The labels come in pairs. A position opened with BTO is later ended with STC (or it is exercised or expires). A position opened with STO is later ended with BTC (or it is assigned or expires). So "sell" on its own is ambiguous: a sell can be someone closing out a call they bought, or someone writing a brand new call. The second half of the label is what separates the two.

Why the opening side matters: open interest

Open interest counts contracts that are open, meaning not yet closed, exercised or expired.² Opening and closing trades are what move it.

• When both sides of a trade are opening (one party buys to open, the other sells to open), a new contract comes into existence and open interest goes up.
• When both sides are closing, a contract is retired and open interest goes down.
• When one side opens and the other closes, an existing contract simply changes hands and open interest stays the same.

That is also why trading volume and open interest are different numbers. Volume counts every contract that traded that day, whatever its purpose. Open interest is a running total of positions still outstanding. Our guide on options volume vs open interest compares the two in more detail.

The worked example: a call bought to open

As of 2026-10-06, at 11:42 ET, the CaymanBot options flow tape showed a call on WMT with a 115 strike, expiring 2026-11-20.³ The data feed labelled it BTO, buy to open. It printed across 89 trades at $1.52 per share, for total premium of $16,000,558, with the stock at about $106.34.

This was someone else's trade on the public tape. The BTO label comes from the data feed; nothing in the data shows who the buyer was, what else they hold, or why they traded. It is used here because it shows the opening side in action.

Step 1: what one contract cost

An option price is quoted per share, and one standard contract covers 100 shares (the contract multiplier). In dollars:

Step 2: how far the strike sat from the stock

A call whose strike sits above the stock price is out of the money. At the print, the 115 strike sat above the stock price of about $106.34, so the stock would have needed to rise before the call had any value at expiration. Our earlier post on why a call can expire worthless after a 7% rise walks through that gap and the break-even on its own.

Step 3: what the BTO label adds

Without the label, the line on the tape would read "a 115 call traded at $1.52, total premium $16,000,558". That fits two very different situations:

1. A holder who bought these calls earlier is selling them (STC), and the buyer on the other side is opening a new long position (BTO).
2. A buyer is opening a new long position (BTO), and the seller is writing new calls (STO).

The BTO label says the buying side was opening. In the first case open interest would stay the same, because existing contracts changed hands. In the second case new contracts were created and open interest would rise. Comparing the contract's open interest the next day with the day before is the usual way to see which of the two happened, and that number is published separately from the trade itself.

What a label can and cannot tell you

A label describes the trade. It does not describe the trader's view.

It can tell you whether the trade started or added to a position, or ended or reduced one. For someone reading flow, that separates fresh positions from positions being wound down.

It cannot tell you what the trader expects. A buy to open on a call is often read as a bullish position, yet the same contracts might be one leg of a spread, a hedge against a short stock position, or part of a larger book that the tape never shows. A sell to close on a call can mean someone is done with an idea, or that they are rolling into another strike. The delta of 0.25 on this contract describes how its price reacts to the stock, and it says nothing about intent either.

It is a label from the feed. Opening and closing flags on the tape are reported by the data source. Our data methodology page explains how CaymanBot sources and presents trade data.

When you read prints, our guide on how to read options flow shows where the label sits next to the strike, expiry and premium on each line.

How to practice this

The labels become clear once you have placed each one yourself. In a paper account, buy to open a call, watch it appear as a long position, then sell to close it and watch it disappear. Then try the other pair: sell to open, then buy to close.

CaymanBot gives you a $100K paper trading account, so you can run through all four labels with no real money at stake.

In the first two Learning Mode stages, each trade on the feed comes with a plain-English sentence you can check against the strike, expiry and premium.

A free account shows options data delayed 16 minutes (60 minutes without an account). Founding Premium costs $119 a year or $14.99 a month for the first 100 paying members, locked while your subscription renews; the regular price is $24.99 a month or $224.99 a year. Premium adds real-time options flow for users who complete the OPRA non-professional attestation. New accounts start with a 14-day Premium trial.

Educational only, not investment advice.