You could demonstrate the combustion easily enough.
Put a small amount of oil on a surface and then set a lit cigarette on top of it.
It will probably not ignite. If it does, find a less combustible oil, and start over.
Next, bring a cutting torch over, turn on the Oxygen only, and direct the flow of Oxygen out of the torch over the small amount of oil in the vicinity of the burning cigarette.
The cigarette will provide the spark needed by the Oxygen, and the oil will provide the fuel.
As an aside: If you have young welders that get hot and sweaty in their coveralls, do not let them blow oxygen into their damp coveralls to cool off. One bad spark and all of their skin would be crispy.