1) MO - modus operandi (as everyone above said) or "mode of operation" (in English vs. Latin)
http://www.merriam-webster.com/dictionary/modus+operandi
http://en.wikipedia.org/wiki/Modus_operandi
2) That is a radio/scanner code. It is how they communicate over their radios - each crime has its own code. Are you sure it is a 419? That one doesn't seem to exist. Anyway, here is a list of police radio and scanner codes. (The scanner ones, which I think are the ones you'd be most likely to hear, are on the right, partway down.)
http://www.radiolabs.com/police-codes.html
3) Yep, Dead on Arrival.
http://acronyms.thefreedictionary.com/DOA
4) Here are some helpful terminology/glossary sites. The first two are police related. (For the second one, you have to skim down, as the cop terminology is the second section on the site.) The second two are forensic/crime scene related.
http://members.aol.com/Welts23/PoliceGlossary.html
http://www.tvacres.com/languages_law.htm
http://library.thinkquest.org/04oct/00206/text_glossary.htm
http://groups.msn.com/ForensicScienceforStudents/forensicterminology.msnw