What does "O I" mean in the context of detected molecules? I've recently been analysing the Exoplanet Database, and have been looking at the 'molecules' field (molecules detected on the planet or in its atmosphere). They all make sense apart from "O I". It's not iodine oxide (which would be I2O), so I'm at a loss as to what it could be, especially since I'm very new to this field.

Any help is very much appreciated!
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.