The issue of oxalates in connection with urinary stones is generally overplayed. Like most herb related toxicity possibilities, problems almost always are the result of extreme excess. If an animal already has oxalate crystals then it would be a good idea to consider low oxalate options. Spinach is very low in oxalates which taste tangy, like lemon. Rhubarb is very high.
Sheep sorrel is somewhere in between.
Where Essiac is indicated, at least in our minds, the issue of oxalate crystals relative to long term use, seems way down on the list of health concerns. If oxalic acid was as big a problem many of the books make it out to be, there would be many humans and animals that have developed side effects. Essiac is in very wide use and has a well-established track record for safety. Our appreciation and thanks for their prompt replies in spite of the disaster. As requested, their thoughts about sheep sorrel and turkey rhubarb will be respected as opinion only, not taken as established fact.