Quote from Robert Weinstein:
Thanks John
It is interesting that based on that website the OCC is jointly owned by the exchanges and they set a time limit of 4:30P CT as a cutoff time to exercise an option that does not expire until Saturday
Your link website also says that the OCC is the counter party to all transacations (much like futures) so perhaps as the counter party they legally could exercise and arb it with afterhours trading any option that went into the money that was not exercised already. since the exchange members appear to own the OCC they would be able to
make a lot of risk free gains
As I recall options used to expire on Friday and they changed it to Saturday so why only change the time of being able to exercise to 4:30 on Friday?
It may be true you can't exercise beyond 4:30pm ET on Friday if you are long an option. This doesn't mean that you are safe from IB or some big players exercising an option on you on Saturday if you wrote an option. This happened to me before.
If I really do not want an option put to me, I close it for pennies on Friday even it is slightly out of the money.