"how can i record multiple channels from a yamaha o1v96 with midi out into my mac with garageband. And be able to mix later."
How many discrete channels do you want to record at once?
I think GarageBand's "limit" is 8 channels simultaneously, though I haven't used it in a while and the most recent versions of GB may be able to do more.
What kind of outputs does the Yamaha mixer have?