"It is snowing."
You can't use it in this sentence because 'o' is a pronoun.
"O kar yağıyor" would mean a specific thing is snowing, or "(S)he is snowing." It usually won't make sense, but I think you could use 'o' if you were referring to a snowglobe that has been shaken up and is now "snowing," or if you have a lot of dandruff and your head is "snowing." :P