In his reddit AMA, Joe Belfiore wrote
i have regular 1-1 meetings with my counterparts in Office, Skype, Xbox.
The little bit of jargon there is 1-1 meeting. This is an abbreviation for one-on-one meeting, a common business practice wherein two people, typically a manager and a direct report, have a face-to-face meeting with no one else present. In the case Joe used, the meeting is not between a manager and a direct report but between two peers.
The term is also abbreviated 1:1, which like 1 − 1 also looks like a bit of mathematical nonsense. But it's not zero or one. It's just an abbreviation for business jargon.
Of course, Microspeak isn't happy with the abbreviation as-is. Microspeak takes it further and nounifies the abbreviation. "I'll bring that up in my 1–1 with Bob tomorrow" means "I'll bring that up in my one-on-one meeting with Bob tomorrow."
Note that the abbreviation OOO is not used to mean one-on-one. That abbreviation is used to mean Out of the office, although it is more commonly abbreviated OOF at Microsoft.