0.08mg per half cup serving, but may vary depending on water used.

10mg is the accepted daily upper limit. So 62.5 cups of oatmeal to reach that max safe limit.

Acute poisoning symptoms may appear as low as 10-15 mg/kg, lethal dose might be 32-64 mg/kg.

So 200 to 400 cups of oatmeal per kg you weigh for a lethal dose.