As far as my understanding: Boy-o reflects one's behavior while it can also be used to address a group of males. But Guy seems to be mostly used in addressing the people while it is less used as a behavioral word.
Please suggest what should I use and when.
Here are some examples I thought of:
Usage : Boy-o
1). How's it going boy-o?
2). That particular stare has the boy-o in it. (I think it makes more sense)
Usage : Guy
1). He's a nice guy.
2). The kids asked passing workers for pennies for the guy.
Also, I think Boy-o seems to be informal as compared to guy. Looking for an appealing noun to address a group of people.
Boy-o reference: Strangers in Death