Hyphens are generally used when two words combine to form a single adjective before a noun, but "one on" doesn't fit that pattern.
Here are some examples of when you might use a hyphen:
* one-on-one (adjective meaning "involving only two people")
* one-sided (adjective meaning "unequal")
However, in phrases like "one on one" or "one on the other hand," the words are separate and don't need a hyphen.