Another corpse was found shot in the head a short distance away from the truck near the town of Santiago Juxtlahuaca, the office said in a statement.
The town is about 93 miles (150 km) west of state capital Oaxaca City.
The statement said none of the bodies had been identified and gave no indication as to a possible motive.
Tens of thousands of Mexicans have been killed in drug-related violence over the past several years, but few such killings have occurred in Oaxaca.
President Enrique Pena Nieto has pledged to reduce Mexico's high murder rate, but violent crime remains at similar levels to those under his predecessor.