I have recently been trying to learn my way around oscP5 and osc in general. My use has so far been limited to sending numerical data but in wading through the example code and doing my problem solving, it seems like what you want to do is send your string as part of the datagram, and extract it at the receiving end.
The oscP5 example code uses
myMessage.add("some text"); /* add a string to the osc message */
and also provides examples of parsing code to recognize and extract what you want.
Would this do what you need?