“In summary, enormous doses of Delta 9 THC, All THC and concentrated marijuana extract ingested by mouth were unable to produce death or organ pathology in large mammals but did produce fatalities in smaller rodents due to profound central nervous system depression.
The non-fatal consumption of 3000 mg/kg A THC by the dog and monkey would be comparable to a 154-pound human eating approximately 46 pounds (21 kilograms) of 1%-marijuana or 10 pounds of 5% hashish at one time. In addition, 92 mg/kg THC intravenously produced no fatalities in monkeys. These doses would be comparable to a 154-pound human smoking at one time almost three pounds (1.28 kg) of 1%-marijuana or 250,000 times the usual smoked dose.”
So in short yes it is pretty much impossible to OD.