You'll also find "Encore On Demand" in the "iO Value" section (which is included in the silver & gold").
It is also available at a charge, if you get the package in an a la carte, or have an old plan (which, if you have the old version & interest in OD, you should call & have it changed - may have to wait until any promo pricing expires).
I was gonna post that after your first post yesterday but got busy at work. For $89.95 (IO Silver) HBO On Demand is included for free as well as most of the others. For $109 (GOld) it includes Cinemax on Demand.