M O is an abbreviation for Modus Operandi. which is Latin for Method of Operation or Mode of Operation
In criminal investigations, it refers to the criminal's methods whether murder or burglary or whatever as similar methods used in separate offences which could indicate the same person was responsible.
It doesn't have to pertain to crime. It can also be used to describe a person's 'trade mark' habits.
My girlfriend always fingers my a-spot when she gives me head. That's her M O.