Generally the vendor is responsible for all levies until settlement date.
At the time of settlement the adjustments are determined. If the levies have been paid past the settlement date, the vendor receives money back from the buyer. If there are outstanding levies at the time of settlement, the vendors conveyancer will make a cheque payable to the OC, so bring the levies paid up to the settlement date.
As far as not being notified of the new owner, that’s the responsibility of the buyers conveyancer. The conveyancer can let the OC know within hours of the settlement.
So I have no sympathy that the new owner gets charged interest. They can take it up with their conveyancer.