First of all, let us talk about the term "hold on." Hold on is more of an idiomatic expression. It can be used in a sentence to mean many things, but the way it is being used would give us a good idea of what it means in a sentence.
Hold can mean to grip or grasp something, to wait or be patient, to keep, and probably more. "Hold on" is not a single word, though, but it is an expression that deals with action, hence, making it a verb. On the other side of the terms, hold off is also an idiomatic expression.
Though it is not a word, the combination of the two words "hold" and "off" gives an action. Hence, making it a verb. "Hold off" in a sentence can basically mean "to repel" or "to delay" or "to keep at a distance", and similar others.