OEX options are American style and trade on the Friday while SPX options stop trading the Thursday before expiration. OEX uses the last closing reported price of each stock in the index on Friday.
SET is more volatile on witching days and it is not rare to have SET be higher than even the high for the index for the morning. So even if the index opens up and runs 8 points, SET could be 15. And if SET is that high you have no defense because the options stopped trading. When you are 10 points or closer on a witching day with significant news to be announced on Friday morning, then you get out on Thursday automatically.