Kalikahuna, SPX options expire on a friday, however, thursday is the last trading day so usually when counting days to expiration thursday is used rather than friday, as it is the case with equity options which expire on a saturday, but friday is the last trading day. I doubt you can hold them liable for that as it is your responsibility to know the product you trade. The SET is calculated using the opening prints for each and every individual stock that constitute the S&P 500 index so the SET value has very little to do with the opening price for the index (i.e. different calculation method). The reason being that stocks post their individual opening prints at different times, some may print at 9:30 others as late as 9:45 or even later. That is why the SET value can be so different from the index Thursday closing value and the index Friday opening value. Once again, you must understand the product you trade!
This is why OX has such high commissions - they have to pay people to deal with newbies like this. You are paying all kinds of staff and managers to spend hours on the phone arguing with people like this much as occured on this forum.
Seems like you can leave yourself wide open for a "gap" on the SET. Can the OP have put in an order to close the position at Thursday's market close?
Yes, there's significant gap risk and, yes, you can easily close out the position at thursday's close.