It seems that there has been much research on the subject of the word "Yo!" Here's some information on the origin and usage of the word!
The following is what I found at the source website. (Their wording - not my wording!)
Yo is an American English slang interjection. The origins of the word may possibly be traced back to 14th century England. However, it was highly popularized after being commonly used among Italian Americans and African Americans in Philadelphia, Pennsylvania.
One of the earliest documented uses of the word in its modern context was in the early 1960s by Philadelphians, especially those of Italian and African descent.
Another common user of the word was Goober Pyle, who was known to use it in the (late 1960s) American TV sitcom The Andy Griffith Show and its later spin-off series Mayberry RFD. It is used in various ways in English, as in when answering a roll call, as a greeting, or as an exclamation. Yo is an informal way of telling somebody something or trying to get their attention.
It was used a lot in John Wayne's 1950 movie Rio Grande
During the late 1990s and early 2000s, the word became very common in rap songs, often chanted in the background between lines, tying in with the hip-hop and gangsta theme. For example: 'yo, check it out, yo'.
A parody of such usage forms the basis for a comic exchange between the animated characters played by Robert DeNiro and Martin Scorsese in Shark Tale (2004), which, along with other demonstrations of its use, can be viewed here.
The word's origin clearly predates these recent uses, as is proven by the words of Rocky Balboa (played by Sylvester Stallone).
Usage -- Here are all of the many uses of the word!!
"Yo" is also often interchangeable with the word "hey," as in "Yo, what's up?" or, "Yo! Wait for me!" or, "Yo, it's me, Smith in the hood, yo!" or, "Yo! pass me the coke!" or, "Yo! homie!", or "Yo what is like chillin!". While it can stand alone as a greeting, like the word "hey", it has a wide range of other meanings that depend on the tone, context, and situation. For example:
If someone is bothering another person, "Yo!" becomes the equivalent of saying, "Hey! Stop it," or "Knock it off!"
If someone accidentally bumps into another person, the expression "Yo!" could be interpreted as "Watch it" or "What in the world!"
Yo is commonly used as a way to get someones attention.
Another way of saying hi or hello: "Yo!"
If someone does something that amazes or shocks another person, the word "yo" (usually "Yo, yo, yo!") is like laughing or an expression of amazement.
If someone says something that's the wrong thing to say, someone might say "yo" meaning "not cool," or "you're out of line.
"Yo" may also refer to another person/thing, mainly to get the attention of that being.
"Yo" can also be used as a substitute for the word "you" e.g "hey you" becomes "hey yo". In the traditional Midlands English dialects of England, 'Yo' is used almost exclusively in place of the standard English 'You'. However, the use of "yo" and other aspects of dialect in the Midlands is becoming increasingly less used.
"Yo" is often used in the Southern United States as a substitute for the response "Here" to acknowledge one's presence during a roll call.
"Yo" has also come to be used as an exclamation at the end of a sentence, either to direct focus onto a particular individual or group ("What's up, yo?"), or to strengthen meaning to a particular point e.g.: "This music is GREAT, yo!" Coincidentally, the Japanese language sentence-final particle yo has approximately the same meaning, but is linguistically unrelated.
"Yo, son!" has also become a widely used phrase, primarily around the North east area of the US. This can be used in place of "Hello, friend!" or "Hello, neighbor!
"Yo" is often used as a short-hand or slang term for "Your", and is used commonly in "yo' momma" jokes, e.g.: "Yo mama so fat, she sat on a piece of coal and it turned into a diamond!" or "yo' mama so old she fart!"
"Yo" is often used to say hi to a close friend or relative if you are just walking by them or if you are giving them a quick talk.
"Yo" could also be used as an exclamation of amazement, surprise or thinking something is cool.
^ Dalzell, Tom (1996). Flappers 2 Rappers: American Youth Slang. Springfield, Massachusetts: Merriam Webster. ISBN 0-87779-612-2.