However, it's not a simple matter as the external supply of oxygen. Chemical reactions have their own way of extracting the oxygen they need. Explosions rely upon oxygen released from nitrates or potassium substances, usually potassium nitrate, potassium phosphate, or nitrogen itself. These readily-available substances are called oxidizers . Obvious arsons are in the last category.
All fires burn upward in an inverted conical shape.
The point of origin is usually therefore the lowest place. Track from least damage to most damage. Theorize, calculate, and estimate time from ignition to flashover. You want to get some estimates of the volume of heat. Odors are more important to trace than charred wiring.