'Oz,' an abbreviation of ounce, is not a valid word in Scrabble. Nor is it a valid word in the similar word games SOWPODS and Words With Friends.
The shortest word beginning with "oz" has five letters: "Ozone."
That said, there are a number of two-letter words beginning with "O" or "Z" that are valid in Scrabble. In fact, turning the letters "O" and "Z" around to make "Zo" would be a valid move. This word refers to a Tibetan yak and can also be spelled "Dzo," "Zho," "Dzho" or "Dso."
Knowing such two-letter words in Scrabble can make it very easy to score high points with minimal letters.