Where did the oxygen from Venus's lost water go?
In plain words
If Venus once held much water that sunlight split into hydrogen and oxygen, the light hydrogen escaped to space but most of the heavier oxygen could not. Venus's atmosphere now has almost no free oxygen, so it must have gone somewhere.
Precise statement
Present O+ escape rates measured from orbit (of order $1e24\ \text{to}\ 1e25\,\mathrm{ions/s}$, approximate) remove over $4\,\mathrm{Gyr}$ many orders of magnitude less oxygen than an Earth ocean contains (about $1.2e24\,\mathrm{g}$ of oxygen in $1.4e24\,\mathrm{g}$ of water), and present atmospheric O2 is below the ppm level. Determine the oxygen sinks: drag-off with hydrogen during an early hydrodynamic escape phase, oxidation of a magma ocean or crust, and consumption by reduced volcanic gases, and hence the maximum water inventory Venus could have lost. An answer is an oxygen budget consistent with the atmospheric $D/H$ ratio, noble gases and the oxidation state of surface rocks.
What would settle it
Descent-probe measurements of noble-gas and oxygen isotopes and of surface-rock oxidation state, combined with escape and magma-ocean models.