What does "O I" mean in the context of detected molecules?
This is the standard astronomer notation for neutral oxygen. Where chemists would write
$\mathrm{O}$ (or $\mathrm{O}^0$),
$\mathrm{O}^{+}$,
$\mathrm{O}^{++}$, and
$\mathrm{O}^{+++}$ (or $\mathrm{O}^{3+}$),
for neutral, once-, twice-, and thrice-ionized oxygen, respectively, astronomers almost exclusively write
$\mathrm{O}\,\mathrm{I}$,
$\mathrm{O}\,\mathrm{II}$,
$\mathrm{O}\,\mathrm{III}$,
$\mathrm{O}\,\mathrm{IV}$, etc.

The same is of course the case for other elements. For instance, you may have an $\mathrm{H}\,\mathrm{II}$ region (i.e. ionized hydrogen) surrounding hot stars, in a larger $\mathrm{H}\,\mathrm{I}$ (i.e. neutral) envelope.

In other words, we start at 1, where chemists start at 0.