Oxalic acid is a chemical substance that naturally occurs in many dark green leafy foods such as spinach and collard greens. When it is ingested, the chemical properties combine with metals in the human body, such as calcium and magnesium. When this happens, it forms a salt. As it has no nutritional value, it will be eventually removed from the body via urination. One can remove oxalic acid from food before eating to avoid excess salt in the body.
Mash your foods that contain oxalic acid before cooking them. This will release the oxalic acid, which will then be cooked out of your food.
Cook your leafy green vegetables. Cooking your greens helps to break down and release the oxalic acid in them. The longer you cook them, the more oxalic acid will be released.
Add calcium carbonate to your greens while cooking. Cooking itself does release much of the oxalic acid, but adding calcium carbonate while cooking removes even more. The calcium carbonate combines to the oxalic acid and removes it from the food. Very little calcium carbonate is needed to make it effective. One tsp. per pot of water is plenty for these purposes. You can buy calcium carbonate powder from most health stores or even online health food retailers.
Although oxalic acid can be a toxin, you would need to eat extremely large amounts of vegetables that contain the substance before it would cause any health issues.
If you grow your own oxalic-acid rich vegetables, fertilize them with a nitrate-based fertilizer. The nitrates break down the oxalic acid in the vegetables. Make sure that while you are providing nitrates, to also provide plenty of oxygen by aerating the soil, and if the plant is covered, allowing it time in open air.