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Offer (ask) - The rate at which a dealer is willing to sell a currency. See Ask (offer) price
Offsetting transaction - A trade with which serves to cancel or offset some or all of the market risk of an open position.
One Cancels the Other Order (OCO) - A designation for two orders whereby one part of the two orders is executed the other is automatically cancelled.
Open order - An order that will be executed when a market moves to its designated price. Normally associated with Good 'til Cancelled Orders.
Open position - An active trade with corresponding unrealized P&L, which has not been offset by an equal and opposite deal.
Over the Counter (OTC) - Used to describe any transaction that is not conducted over an exchange.
Overnight Position - A trade that remains open until the next business day.
Order - An instruction to execute a trade at a specified rate.