Ounces to grams
One ounce is exactly 28.349523125 grams — an exact figure, because the pound is defined as exactly 0.45359237 kilograms and an ounce is a sixteenth of it. For cooking, 28.35 is plenty and most people use 28.
How to convert ounces to grams
The ounce you almost certainly want is the avoirdupois ounce, used for everything from flour to parcels. Two others exist and both cause trouble. The troy ounce, used only for precious metals, is 31.1 grams — about ten percent heavier, so a gold price quoted per ounce is per troy ounce. And the fluid ounce is a volume, not a weight, so converting fluid ounces to grams is meaningless without knowing what the liquid is: a fluid ounce of water is about 29.6 grams, of honey nearer 42.
Questions
Exactly 28.349523125, which follows from the pound being exactly 0.45359237 kg. Use 28.35 for cooking.