Here's why:

* Oil is a fuel: Oil is a hydrocarbon, which means it's composed of hydrogen and carbon. It can burn, but it needs a source of ignition and a good supply of oxygen to do so.

* Oxygen is an oxidizer: Oxygen is necessary for combustion, but it's not flammable itself. It acts as an agent that helps the oil burn.

Here's what makes things explosive:

* Fine mist/aerosol: When oil is broken down into a fine mist or aerosol, it becomes much more volatile and can ignite easily. This is why oil spills or leaks can be extremely dangerous, especially in enclosed spaces.

* Confined spaces: In a confined space, the concentration of oil vapor can reach a level where it can ignite explosively.

* Heat and spark: A source of heat or spark can ignite the oil vapor, leading to an explosion.

In conclusion, oil and oxygen alone are not explosive. However, the presence of oil vapors in the air, a source of ignition, and a confined space can create a highly dangerous situation.