Answer: 4/3x (or 1/3 + x, since your wording's pretty odd)
Explanation:
i can't tell if the one-third you're referring to is a third of how many ounces of juice you drank (meaning it will change if you drank 1, 2, 3 oz of juice and so on) or if it's always going to be a third of an ounce more than the amount of oz of juice you drank (in which case it'd be 1/3 + x). anyway just choose whichever answer you're looking for :3