One cancels the other (OCO)

A link between two working orders that makes them mutually exclusive. It appears in two familiar shapes. The first brackets an open position, pairing a protective order below the market with a target above it, so whichever level the market reaches first ends the position and removes the other instruction. The second brackets a quiet range with an entry on each side, so a move out of the range in either direction leaves one order live and cancels the entry that was never wanted.

The link is maintained by whichever system holds the orders, and that is the detail worth checking in a platform's specification. Where the pair rests on the firm's server, the cancellation happens at the venue whether or not the terminal is open. Where the pairing is maintained by the terminal or by a script running on a local machine, it exists only while that machine is connected, and a disconnection can leave both orders live with nothing linking them.

The link governs cancellation and not execution, which is what readers most often invert. Both orders are genuinely live until one of them fills, so in a fast market both can be reached before the cancellation is processed, and firms document how that case is resolved rather than promising it cannot happen. A partial fill raises the same question in a milder form, and practice differs: some venues cancel the linked order outright once any quantity executes, others reduce it to the remaining quantity, and the two behaviours produce different exposures from the same pair of instructions.