The avoirdupois ounce (oz) is the ounce used for everyday weights in the United States: food packaging, postal items and kitchen scales. NIST lists it as 28.34952 grams, and the avoirdupois pound as 0.4535924 kilograms, so 16 ounces make one pound. It measures mass. The fluid ounce shares the name but measures volume.
Conversion
g = oz × 28.34952
oz = g ÷ 28.34952
Example
A 16 oz bag is 16 × 28.34952 = 453.6 g, which is 0.4536 kg. A recipe flour amount of 120 g is 120 ÷ 28.34952 = 4.23 oz, matching the "4 1/4 oz per cup" King Arthur Baking lists for all-purpose flour.
Use grams to ounces or ounces to grams. The cups to grams guide shows why a cup of an ingredient has no single weight in ounces.
Common mistake
Recipes that say "8 oz" for a liquid usually mean fluid ounces, one cup; "8 oz" of flour on a scale is about two cups. Check which kind of ounce a quantity uses before converting.