One on one? one to one?

What is the difference between *one on one* and *one to one*??
Thanks

From Merriam-Webster:

Main Entry: one-to-one
Function: adjective
1 : pairing each element of a set uniquely with an element of another set
2 : ONE-ON-ONE 2

Main Entry: one-on-one
Function: adjective or adverb
1 : playing directly against a single opposing player
2 : involving a direct encounter between one person and another