Buy to Close

Buy to close is an options order type used to exit a short options position by purchasing back the contracts you previously sold. When you sold those options using a "sell to open" order, you created new contracts and took on obligations to the buyer. A buy to close order eliminates those obligations by purchasing identical contracts, netting your position to zero and closing your account exposure. It is one of the four main options order types alongside buy to open, sell to open, and sell to close.

Think of buy to close like covering a short stock position: you borrowed shares, sold them, and now you are buying them back to return what you borrowed and settle the trade.

You Use Buy to Close After a Sell to Open

The sequence is straightforward. You sell to open a call or put option, collect the premium, and take on an obligation. That obligation exists until expiration or until you close it. If the option's price falls after you sold it, you can buy to close at the lower price and keep the difference as profit. If the price rises, you can buy to close to limit your loss before it grows larger.

The most common practical use is covered call writing. You own stock and sell a call against it to collect premium income. If the stock approaches your strike price and you want to keep the shares, you buy to close the call, accepting the cost, rather than letting the option expire in the money and having your shares called away.

Buy to Close Reduces Open Interest

When you execute a buy to close order, you are eliminating an existing contract from the market rather than creating a new one. This reduces the total open interest in that specific contract. A buy to open order has the opposite effect: it creates a new long position and increases open interest.

Specifying the correct order type matters. Using buy to open when you intend buy to close would create a new long position alongside your existing short position, doubling your exposure rather than eliminating it. Most platforms flag this type of error, but confirming your order type before submission is essential.

Time Decay Determines How Cheaply You Can Close

Options lose extrinsic value daily as expiration approaches, a process called theta decay. When you are short an option, theta works in your favor because your short contract becomes cheaper every day the underlying stays away from your strike price. A buy to close order placed just before expiration typically costs far less than one placed weeks earlier. Many short options traders manage positions by buying to close once they have captured 50% to 80% of the original premium rather than waiting for the contract to expire completely.

Sources:
https://www.optionstrading.org/basics/order-types/buy-to-close/
https://www.sofi.com/learn/content/buy-to-open-vs-buy-to-close/
https://tickeron.com/trading-investing-101/what-buy-to-close/
https://www.gobankingrates.com/investing/strategy/buy-to-close/