http://bioindividualnutrition.com/oxalates-their-influence-on-chronic-disease/ "Oxalates are highly reactive molecules; they present in our body as sharp crystals or crystalline structures with jagged edges that cause pain, irritation, and distress. They can bind with certain minerals; particularly calcium and magnesium, as well as iron and copper. Having high oxalate in the body can be problematic; and not giving proper consideration to one’s oxalate intake can impede the effectiveness of even the best healing diet protocol." "The health of the gut and one’s microbiome influence whether or not a person has an issue with oxalate"
Oxalates don't appear in urine only as calcium or magnesium crystals. A number of OAT tests look for oxalates but only one, from Great Plains Laboratory, looks at two other metabolites, glyceric and glycolic acids, which give insight into possible endogenous oxalate production. Oxalate dumping is not the same thing as passing an oxalate stone. The oxalate anion alone is capable of causing a wide variety of problems and in this form can be dumped via the bowel, kidneys, skin or lungs in an attempt to reduce concentrations of this highly toxic substance in the body. It is carried out of the body by specific transporters in exchange for sulfate. Here is a summary that might be helpful and a list of oxalates in food.
Due to its relationship with calcium it' s potentially problematic for vegans http://www.veganhealth.org/articles/oxalate Some interesting info