What is LOX?
Also called: liquid oxygen
LOX is liquid oxygen — ordinary oxygen chilled until it becomes a pale blue liquid at about −183 °C. Rockets carry it because fire needs oxygen and space has none.
A rocket needs oxygen far faster than it could ever scoop from the air, and it leaves the atmosphere within minutes, so it brings its own in a tank alongside the fuel.
Falcon 9 chills its LOX further still, to roughly 66 K — below the boiling point. Colder liquid is denser, so more fits in the same tank.
That is why fuelling begins only 35 minutes before launch: densified oxygen warms up continuously once loaded.
See it happen
Push & GoWhy rockets flySource: Falcon 9 countdown timeline