The correct translation was given as: "Oxygen as we know it's a gas." This sentence is incorrect because it has two subjects: oxygen and it.
The correct English sentence is "Oxygen, as we know, is a gas". (Unless you're Mick Jagger)
Not quite, that would be "Kyslík, jaký známe, je plyn". That would probably be a more common utterance, but is somewhat imprecise since, like all other elements, it can also be liquid or solid given a low enough temperature.