During expiration week, we often talk about the importance of monitoring option activity in OEX because it can have an influence on the overall market. This is especially important for stock traders and stock index futures traders. Whether you're short-term oriented or merely wanting to know what's going on, you should understand the ramifications of an early exercise of OEX options.
Most stock options and a few index options are American style, meaning that they can be exercised at any time during their life. Most index options are not American style, but are European style, meaning they can only be exercised on the day they expire. Large institutions — many of whom sell index and sector options to hedge portions of their portfolios — prefer European style exercise because they then know they can't be called out of short positions prematurely. However, OEX options have always been American style. This makes them more interesting, but it means that one has to be on his toes when he is trading spreads in OEX — either debit spreads or credit spreads — if short options in the spread become deeply in-the-money.
