What does M.O mean when someone says." That's his M.O"?

It means modus operandi.

Definition:

Mode of operating or working.

The way someone does something; a characteristic method: “Her modus operandi in buying a new car always included a month of research.” This phrase, often abbreviated “m.o.,” is used by police to describe a criminal's characteristic way of committing a crime. From Latin, meaning “method of operation.”

See synonyms for modus operandi free online thesaurus, antonyms, and definitions at Thesaurus.com.

Tags: m.o.modus operandi 
Tuesday, August 08 2017
Source: http://www.dictionary.com/browse/modus-operandi