Oxalates are salts of oxalic acid containing oxalate ion. Oxalate ion is a dianion. Upon protonation, oxalate ion forms a commonly known compound, oxalic acid. The commonly known oxalate salts are sodium oxalate, potassium oxalate etc. The calcium metal ion reacts with oxalate ion to form an insoluble precipitate of calcium oxalate, which is the primary constituent of most of the common kind of Kidney stones.
This video explains how to test the presence of oxalate ion in a given salt.