1. [adv]  (of two persons) in direct encounter; "preferred to settle the matter one-on-one"; "interviewed her person-to-person"
2. [adj]  (sports) being a system of play in which an individual defensive player guards an individual offensive player; "one-on-one defense"
3. [adj]  directly between two individuals; "one-on-one instruction"

ONE-ON-ONE is a 10 letter word that starts with O.