Octane has a boiling point of 120 °C. Water has a boiling point of 100 °C. The definition of boiling point is, "the temperature which the liquid substance's saturated vapor pressure equals the atmospheric pressure". Volatile substances have higher saturated vapor pressure at a given temperature, than the lesser volatile substances. When temperature increased (heated), the saturated vapor pressure increases rapidly and non-linearly. So, the volatile substance's saturated vapor pressure reaches atmospheric pressure quicker than the lesser volatile substance, hence, reaching boiling pint at a lower temperature.
However, water which is a less volatile substance than octane (if you put two separate liquid spots from water and octane, octane evaporates faster) has a lower boiling point than octane at atmospheric pressure. Is there something I am missing? If my reasoning is wrong please correct me.