Are cc bill payments from OCBC 360 a/c for my Citibank Credit Card immediate?

The credit card bill payment usually takes a couple of days for the transfer to go through.

It is not immediate, the fastest I think is 2 business days. Over weekend will be longer. So usually I will pay the cc bill a few days before the date due.

Some cc issuers will not charge late payments fee if the payment comes in after the due date. But it depends.

Either way you could call and request for a waiver since you can prove that you have paid the bill before the due date.