If this happened to me, I would consider it karmic retribution for not carrying my O-ring kit, as I know I should. If I wasn't going to use all the tanks at the same time, I'd just throw out the bad O-ring and replace it with an O-ring from one of the other tanks (as someone basically already said). The O-ring-less valve would be caught and re-ringed when they went to fill it. If I needed all the tanks at once, I'd just pull the O-ring and flip it around. That'll usually get another dive out of it in a pinch, unless it's cut, of course. (Do divemasters not learn that old trick anymore?)
Perhaps I'm considerably more forgiving than many people, but since the fault was my own, at least in part, the buck stops with me. Will I mention my disappointment that I didn't catch the bad O-ring when I go to the shop? Sure. Will I ask them if they ever check the O-rings? Definitely (but in a fairly nice manner, especially since it's a loaded question). Will I suggest it might help other divers (and my occasional forgetful self) if they'd remind people (and me) to check the O-rings when they rent the tanks (so they can swap them out if they look ragged)? Yeah, they'd probably think that's a decent idea.
If I'm at fault for something as basic as an O-ring, I refuse to share that blame with anyone, as I need to learn to be better and pain is an excellent teacher. (Perhaps somewhat strangely, more than once my accepting full blame for something that may have been partially the fault of a shop has ended with them apologizing for their part and giving a refund or credit, but maybe that's what happens when you go in humbly instead of being yet another you-owe-me customer.)