Why Are Police Officers Called '5-o's'?

A television show called Hawaii 5-0. Simply put, Hawaii is the 50th state. In Hawaii, that was the street name for them. Hope this better helps your question.

Calling police officers "5-O" comes from a TV show of about 25 years ago called "Hawaii 5-0."  This show was set in Hawaii, and the unit the show was about was 5-0." From that time until today (and probably forever!) law enforcement officers and correctional officers are referred to by criminals as "5-0."  On the streets and in prison, when people who are up to no good see a police officer or corrections officer coming their way, they softly say "5-0" to alert others that the law is coming their way. This is not an offensive or disrespectul phrase; sometimes law enforcement officers and correctional officers use it themselves.

Comes from old tv show called Hawaii 50.  Another phrase that came from this show was "book 'em danno".

Because it sounds cool and they like to talk in 'code' :)  Also from that TV series mentioned below.
-Rich @ How to become a police officer

Actually the term 50 came from when the police dept started buying the mustang 5.0 back in the late 80's and early 90's. That is when it all started. If someone seen the cops coming they would yell five o