Statement (I) : Reactions of oxygen once initiated easily sustainStatement (II) : Reactions of oxygen are exothermic

O=O bond enthalpy is 498 kJ/mole, so to break this double bond a large energy is required;Thus reactions involving oxygen require energy(heat (or) light) for initiationBut propagation is very easy because reactions of oxygen (except with Nitrogen & Fluorine)are highly exothermic. So once initiated, reactions get propagated easily;