Real Dictionary

y.o. in American English
abbreviation: year old; years old

yo
convention: People sometimes say 'yo' to greet other people or to get their attention.

yo in American English
used variously to attract attention, greet someone, introduce or emphasize a remark, etc.
interjection: (used as an exclamation to get someone's attention, express excitement, greet someone, etc.)

yo in British English
sentence substitute: an expression used as a greeting, to attract someone's attention, etc