I'm cancelling reservations for Ohana on Oct. 27th 5:20pm for six people. Hope this helps someone.
I cancelled one and it showed up instantly online so I would think online may be the best method.
I got mine online after I read a cancellation post. It was within minutes of the post. I also checked online for an ADR I was considering from a cancellation post and it had been at least an hour and it was still there. I think it really depends on the popularity of that restaurant. Good Luck !
In my case I did it online.