What's the difference between magnesium oxide and magnesium citrate?

Different mag salts. You cannot take magnesium alone. It has to be compound with something like oxide, citrate, or sulfate. We call these salts of magnesium. Like the salt of table sodium chloride. But you can take sodium in other forms like sod bicarb (in baking soda). Mag citrate causes diarrhea that's why it is used for constipation. Also, good for preventing kidney stones. Mag ox is used for repleting low mg level.