By exercising what you have purchased, you are choosing to take delivery of (call) or to sell (put) the underlying asset at the option’s strike price. Only buyers have the choice to exercise an option. Sellers, on the other hand, may experience having an option assigned to a holder and subsequently exercised.
Offsetting is a method of reversing the original transaction to exit the trade. If you bought a call, you have to sell the call with the same strike price and expiration. If you sold a call, you have to buy a call with the same strike price and expiration. If you bought a put, you have to sell a put with the same strike price and expiration. If you sold a put you have to buy a put with the same strike price and expiration. If you do not offset your position, then you have not officially exited the trade.
If an option has not been offset or exercised by expiration, it expires worthless. If you originally sold an option, then you want it to expire worthless because then you get to keep the credit you received from the premium. Since a seller wants options to expire worthless, the passage of time is a seller’s friend and a buyer’s enemy. If you bought, the premium is nonrefundable even if you let the it expire worthless. As it gets closer to expiration, it decreases in value.
It is Important to note that most options traded on u.s. exchanges are American style. In essence, they differ from European options in one main way. American style options can be exercised at any time up until expiration. In contrast, European style options can be exercised only on the day they expire. All the options of one type (put or call) which have the same underlying security are called a class of options. For example, all the calls on ibm constitute a class. All the options that are in one class and have the same strike price are called a series. For example, all ibm calls with a strike price of 130 (and various expiration dates) constitute a series.