Chemistry equation

Hey hey,
Just a question on the balanced equation of magnesium.
I thought it would be Mg + O ----> MgO
but it's not, and instead it says it is 2Mg + O2 ----> 2MgO

I don't get why, like in both cases they are balanced, no? And why are we adding a random 2 in the equation. I'm confused.....


Thanks for helping meeeee~

Oxygen in gaseous state is always a diatomic molecule just like other halogens. So it has to be O2 not O