How did the Apollo program measure Oxygen Quantity? The Command Module had a Pressure Gauge and a Quantity Guage for the oxygen bottles. Measuring pressure is simple, but I don't see how quantity is measured in a low gravity environment.
Assuming you are asking about the oxygen in the Service Module tanks - the stuff used as a reactant in the fuel cells - quantity was measured by means of capacitance probes in the tanks.
(Source: Apollo Experience Report - The Cryogenic Storage System - I added the red arrow)
This only worked because the oxygen was stored as a supercritical fluid. If you are not familiar with that, please read this answer:
The key takeaway from that answer that applies to the quantity measurement question is that the supercritical fluid is "gas-like" in that it will occupy the entire volume it is contained in.
The gauge measures the density of the fluid. Since the supercritical fluid completely fills the tank, and the tank volume is known, if you have the density, you can calculate the mass of the fluid.
As Wayne Conrad commented, the famous "tank stirring" plays into this. Stirring the tank kept the reactant mixed up so that its properties would be uniform throughout. An unstirred tank in free-fall conditions tends to "stratify" - develop density gradients related to the distance from the heater elements.
More on this stratification effect here:
(I thought quantity gauging was covered in one of those "supercritical fluids" questions back in 2019, but I only see it mentioned in comments. If I missed it and this is a duplicate, please VTC.)