Could oxygen start a fire

Not on its own. To have a fire you need three ingredients: fuel, oxygen, and heat. Fire is a chemical reaction between oxygen and some flammable fuel. The heat is needed to ignite and sustain the reaction. Our atmosphere is 21% oxygen, which makes fire possible here. Oxygen tanks, which contain pure oxygen, are a fire hazard because higher concentrations of oxygen make it easier for flammable materials to ignite and allow a fire to burn hotter and spread faster.