This pipe was used to vent excess fumes from the interior of the tank, to help prevent the risk of explosion.  Additionally, this pipe also allowed airflow into the tank aiding in the conveyance of oil to the burner through the feeder lines.  The vent pipe is usually vertically attached to the house and slopes to the tank on the opposite side of the tank than the filler pipe.  The vent pipe is usually 1-¾ inches in diameter and has a open cap on the top of it.  The cap on top of the vent pipe blocks water from entering in the tank and allows the air to flow freely without interruption.