Trader Tip. The ticker symbol for the European Style S&P 100 Index (.XEO) is OEX backwards. The .OEX is the ticker symbol for the S&P 100 Index (.OEX). What’s the difference? OEX options are one of the few cash indexes that settle American Style, which means that exercise and assignment can happen at any time prior to expiration. The XEO settle European style, which means that assignment is only a risk at expiration. Exercise isn’t possible prior to expiration.