DE - for elements, molecular weight and atomic weight are both acceptable, and interchangeable. You are correct that 'atomic weight' is the more precise term.
Free radical O does exist in nature, as does C. However, a molecule, by definition, is a compound of two or more atoms - so yeah, I should have used the term 'atomic mass' or 'atomic weight' for C and O,and then thrown in 'molecular weight' for only 'octane'. Would have had to include a note that molecular weight and atomic weight are measuring the same thing - the total weight of the atom and the total weight of atom(s) when in a compound. You have now made that point.
There are several known allotropes of oxygen. The most familiar is molecular oxygen (O2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O3). Others include:
Atomic oxygen (O1, a free radical).
Singlet oxygen (O2*), either of two metastable states of molecular oxygen.
Tetraoxygen (O4), another metastable form.
Solid oxygen, existing in six variously colored phases, of which one is O8 and another one metallic.
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