Why is the word OF different on the 1976 us quarter?
It's purely a particular artistic style. Because the word "of" is considered to be a minor preposition it is often not capitalized in titles and headlines. Sculptors and inscribers sometimes morph that idea into the peculiar-looking combination of a small "o" and large "F". The practice was more common in the early 20th century, along with using the Roman alphabet that substituted the letter V for U*. See for example the lettering on a $2.50 Indian Head gold piece and on the famous St. Gaudens $20 coin.
(*) There's a very long thread where several of us try to convince posters that "In God We Trvst" is NOT a typographical error!!