Opening vs closing option transactions

Every option trade opens or closes a position for each of its two parties: buy to open, sell to open, buy to close or sell to close. Public trade reports do not say which, so flow readers infer it from the next day's change in open interest or from a trade's size relative to existing open interest.

Senzoukria · Glossary · Updated September 2026

At a glance

Four actions
Buy to open, sell to open, buy to close, sell to close
OI rises by one contract
When both sides open
OI falls by one contract
When both sides close

How open interest moves

Each traded contract has a buyer and a seller, and each may be opening or closing. If both open, open interest rises by one; if both close, it falls by one; if one opens and the other closes, the position changes hands and open interest is unchanged. Open interest is published after the session, so the net effect of a day's trading only becomes visible the next day, and one number summarises thousands of trades.

Worked example

A call series had 1,200 contracts of open interest at yesterday's close. Today a single print of 3,000 contracts crosses. Even if every existing holder closed, at most 1,200 contracts of that print could be closing trades from yesterday's positions, so a large part of it must open new positions: the volume-to-open-interest ratio of 2.5 makes an opening trade very likely. If tomorrow's open interest is 4,000, the day added 2,800 contracts net, consistent with that reading.

  • Volume above prior open interest on a series suggests opening activity.
  • A large print with no change in next-day open interest suggests a transfer or a close.
  • Intraday positions opened and closed the same day never appear in open interest.

Limits of the inference

  • Positions opened earlier in the same session raise the true open interest before it is published, so closing volume can exceed yesterday's figure.
  • Open interest changes are netted over all trades of the day; they cannot be attributed to one print.
  • The side of the trade (who was the aggressor) and the opening or closing status are separate questions; a buy at the ask can be a buy to close.

In Senzoukria

Option Flow attaches to each print the open interest of its contract from the latest chain snapshot and shows a Vol/OI column: the print's size divided by that open interest. At 1x or more, the print is tagged OPEN, because a single trade larger than the standing open interest cannot be a simple rotation of existing positions. When open interest is unknown or zero, for instance for a contract listed that morning, the column shows a dash instead of an infinite ratio. The top contracts list flags contracts whose cumulative volume exceeded their open interest. Open interest itself is the figure published after the previous session.

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Frequently asked questions

Can I know for sure whether a trade opened a position?
Not from public data. The broker knows it for its own clients, but trade reports do not carry it. Next-day open interest and size relative to existing open interest are the usual clues, and both can mislead.
Why does opening versus closing matter?
Because a new position represents fresh risk someone chose to take, while a closing trade removes risk. The same large call purchase means something different if it opens a speculative position or closes a short call that was hedging stock.

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