The acronym M.O. stands for 'modus operandi, ' which is a Latin phrase meaning 'method of operating.' It is commonly used to refer to the particular habits or practices of an individual or group, especially when they are criminal in nature. The crossword solutions for M.O. are STYLE (5 letters) and SYS (3 letters). STYLE refers to the distinctive way in which someone carries out their activities, while SYS is short for system, which is a set of related elements that work together to achieve a common goal.