"In Defense of Oxalic Acid". An article describing its benefits and risks relating to the formation of kidney stones and the absorbtion of calcium. Includes a food chart with oxalic acid levels. A common source is rhubarb. A very good read!
Amaranth is an edible weed. You can eat all parts of the plant, but be on the look out for spines that appear on some of the leaves. While not poisonous, amaranth leaves do contain oxalic acid and may contain large amounts of nitrates if grown in nitrate-rich soil. It’s recommended that you boil the leaves to remove the oxalic acid and nitrates. Don’t drink the water after you boil the plant. With that said, you can eat the plant raw if worse comes to worst.