Why are O-O bonds so weak? Good question. I think this is part of what Pauling was attempting to explain when he created electronegativity theory. If you look as homolytic bond strength, I-I<Br-Br<Cl-Cl>F-F. Why are I-I bonds weak and why aren't F-F bonds stronger? From bond energy, more energy is released upon breakage of the bonds of the smaller atoms. If the bond is also weaker, fluorine is very reactive. I expect oxygen should be similar. At this point, I find it easier to simply remember this than to provide a universal principle that can explain it.