Though note that etymologically "of" is a derivative of "off". There was no need for a preposition "of" historically because the idea was expressed by declining a noun into the genitive case. But as the Romance languages developed from Latin, the case system largely died out and the Latin preposition de radically shifted its meaning to the current meaning of "of".

Romance de was translated into English as off because that is the appropriate translation of Latin (but not Romance) de.[1] And, the senses being unrelated, of diverged from off.

[1] Actually, the conventional translation my Latin instruction used for de as a verbal prefix was "down from". (As in "descend".) As a preposition it has a few different uses, including "off", "down from", and "about" (as in "what's that book about?"). "From" more generally would be e or ex, but "down from" gets its own preposition.

Noting of course that “off of” is mostly an American-English construction (although it does also pop up in some of the more ‘working class’ British dialects like cockney).