i got something similar working a bit ago...
touchOSC to processing -> use the plug functions to connect the incoming OSC messages to a function inside of processing. once you have the functions and messages connected, you can call serial commands to send messages to an arduino. you'll have to build a simple protocol to send messages and have commands. like @r025,g056,b128+ where the '@" is the packet beginning, and the '+' is the packet ending.
on the arduino side, if you have the blinking LED going, you have all the connection stuff working, just parse your packet that you sent, and apply it to functions inside your arduino code.
don't expect blazing fast reactions... there are a lot of little steps happening, all the can be optimized, but are outside my skills... (if you get to the point of making it super fast let me know)
hope some of this makes sense and is helpful.