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onto vs. one-to-one

onto

[on-too, awn-, on-tuh, awn-] / ˈɒn tu, ˈɔn-, ˈɒn tə, ˈɔn- /

preposition

to a place or position on; upon; on.

to get onto a horse.
Informal. in or into a state of awareness about.

I'm onto your scheme.

adjective

Mathematics. Also pertaining to a function or map from one set to another set, the range of which is the entire second set.

one-to-one

[wuhn-tuh-wuhn] / ˈwʌn təˈwʌn /

adjective

Also (of the relationship between two or more groups of things) corresponding element by element.
one-on-one.