How did ounce come to be abbreviated as oz.?

The word ounce ultimately comes from the Latin word uncia, meaning "one-twelfth." (This is also where we get the word inch.) In Italian, this became onzo, which is where we get both the word ounce and its abbreviation, oz.

"But," you say, "there are 16 ounces in a pound, not 12!" It's true that in the international avoirdupois system, 1 pound equals 16 ounces, but this wasn't always the case. In ancient Rome, a pound (libra in Latin, which is where the abbreviation lb. comes from) was about 12 ounces. Even today, jewels and precious metals are measured in troy ounces (which are heavier than avoirdupois ounces), and there are 12 troy ounces in a troy pound (which is lighter than an avoirdupois pound).