I found this infographic that seems to say that oxygen is the most abundant element on the surface of the Moon. Is this really the case? If so, under what form is this oxygen?
Yes, that's correct; it's also true for the Earth's crust. The reason is that "rocks" are typically made up of components containing combinations of silicon or one or more metals (e.g., magnesium, aluminum, iron) and oxygen, such as silica ($\mathrm{SiO}_{2}$); alumina ($\mathrm{Al}_{2}\mathrm{O}_{3}$); lime ($\mathrm{CaO}$); iron oxide ($\mathrm{FeO}$); and magnesium oxide ($\mathrm{MgO}$).
Examples of common lunar minerals formed from these components includes plagioclase feldspars (mixtures of NaAlSi$_{3}$O$_{8}$ and CaAl$_{2}$Si$_{2}$O$_{8}$), pyroxene (typically XYSi$_{2}$O$_{6}$, where X and Y are metals such as calcium, sodium, iron, magnesium, and aluminum), and olivine (made up of Mg$_{2}$SiO$_{4}$ and Fe$_{2}$SiO$_{4}$), along with oxide minerals like ilmenite (FeTiO$_{3}$). (Source)
Since in all these cases you have between one and two oxygen atoms for every non-oxygen atom, you end up with oxygen as the most abundant single element.
