I don't really understand what your asking here, you want to be able to set the order of the messages in the ringbuffer?

Its a FIFO queue, as in First In - First Out. So the order of the messages is the same as you write them...
Perhaps I'm misunderstanding you :S

W.r.t OSC, there's 2 ways to run an OSC server, polling or having callbacks. If you have callbacks it involves a having a seperate OscServer thread. This means you'll need to use a seperate RingBuffer too, as the JACK ringbuffer is a SRSW queue. (single read, single write)

If you're using the GUI thread to poll the OscServer, then you can use the same RingBuffer and you'll have more control over the order of GUI / OSC events...

Then again, if you can do the "ordering" on the recieving side of the RingBuffers, then it really makes no difference :D
All the best, -H