On reducing harm from oxalates, http://www.lef.org/protocols/kidney_urinary/kidney_health_03.htm mentions consuming:
Citrate: "potassium/magnesium citrate effectively prevents recurrent calcium oxalate stones, and this treatment given for up to 3 years reduces risk of recurrence by 85%" (Ettinger 1997); and
Calcium: "calcium and oxalic acid ... form calcium oxalate inside the kidneys. Oxalic acid is able to pass through the intestinal wall, into the blood, and enter the kidneys where it has a chance to combine with calcium. Calcium oxalate, when normally combined inside the digestive tract, does not pass through the intestinal wall and into the blood, but is eliminated with other waste products. Therefore, when combined with dietary or supplemental calcium inside the intestinal tract, oxalic acid will never reach the kidneys; thus, calcium oxalate kidney stones cannot be formed ... dietary calcium intake from food or supplements reduced the risk of renal calculi; calcium supplementation must be taken with food and in small dosages (< 400 mg)...."