If the o-ring breaks then only a already chambered round will fire. After that you would have to do everything manually.
Call CZ and ask how much o-rings are. Before Armsco closed down, they were selling them for $1 each. If they charge too much (like Remington does for their shotguns), bring one to a good hardware store and ask them if they can tell you the size. These others probably won't last as long, but if the factory is $5+ (again like Remington), a $0.25 one can not last as long but still save money. I would just put in a factory one if you were in a hunting situation. That is never the time to have an o-ring fail! It happened to me and I lost the greates second shot in history: the turkey actually jumped up and flew towards me.. not away, but towards! When it landed it was all of 10 feet from me and I could not get a second shot off!