While cats do use tears to protect their eyes from irritants and dryness, they don't cry to express sadness in the same way as humans.
"It is just eye discharge, which is not uncommon in cats," Mikel Delgado, a Ph.D. candidate and certified cat behavior consultant at the University of California–Berkeley, told The Dodo. "I assume something else is going on with this cat (hence the oxygen) - it could be a response to an irritant or allergen in the air."
But that bit of information shouldn't distract us from what we already know: Just because an animal can't produce tears of sadness doesn't mean he doesn't have those kinds of feelings. Whether you're a human or a cat, we all share a capacity to feel emotions such as happiness, despair and even gratitude.
What's the cat in the video feeling? Perhaps she feels sadness after whatever it is she's been through. Perhaps she feels grateful for being rescued from wherever she was. Perhaps she feels calm, knowing that the worst is behind her and her new life begins today.