If you didn’t know Opal is a “club Apple” - that means it’s propagation is very tightly controlled. So, as a backyard grower it will be nearly impossible for you to get hold of a tree. A few here tried to grow seedlings but I’m not sure what’s the status now.
I’m assuming you don’t want high acid fruits. BTW, at high sugar levels, the high acid fruits develop incredible flavor but that’s not everyone’s cup of tea. For low acid stone fruits, I’d cite the following