The link above didn't mention the LD50 for it.
"LD50 (LD50 is the Median Lethal Dose, which is the dose of a drug or chemical predicted to produce a lethal effect in 50 percent of the subjects to whom the dose is given) in rats is 375 mg/kg. So for a person about 145 pounds (65.7 kg) that's about 25 grams of pure oxalic acid required to cause death."
I'm guessing that 25 grams is about a heaping tablespoon or two.
The Health rating of 4 (cyanide is also 4, I think) is because all the oxalic acid you absorb/ingest will end up complexing with your calcium and will end up plugging up your kidneys. You can not excrete it. It accumulates as insoluble CaOxalate. That is why they rate this as 4, extremely damaging to health. So even if you don't take in the 25G needed to kill you outright, your kidneys might rot with a smaller amount or with repeated exposure.
Yes, it is a weak acid - the chemical term to indicate it does not ionize completely in water, unlike hydrochloric, sulfuric, etc. Acetic acid is also a weak acid. But unlike acetic acid, Oxalic is toxic to the kidneys.
So it pays to warn people when they use this!!! Especially using it around critters to which you do not wish an agonizing death.