The ounce is a unit of weight equal to one-sixteenth of a pound. In the avoirdupois system (used to weigh objects in general) one ounce equals 28.34952 grams. In the troy system (relating to precious metals and gemstones, as well as medicines) the ounce is worth 31.10347 grams. The word ounce originated in the Latin term uncia, which means the twelfth part, that is, `1/12`. This is because, the Roman pound was then divided into 12 ounces. Nowadays in the UK, the pound is divided into 16 ounces.
The Gram is a unit of mass and is one-thousandth of the standard kilogram. Although the kilogram is the fundamental unit of mass, in practice we use gram as the main unit. This is because, the multiples and submultiples of the mass units, are defined from the gram, see the case of hectogram, decagram, decigram, centigram, etc. Did you know that a cap of a pen weighs about a gram and that this is also the approximate weight of a paper money?