Well, you're a bit light on helpful detail here. Don't know what software you're using or what you want to accomplish. But let me take a stab at answering the question first.

The big question is what you want to use for a controller keyboard...the Oxygen or the K1 (or even the KAOS). Whichever you choose would have its MIDI Out connected to the US-428's MIDI In 1, and its MIDI In connected to the 428's MIDI Out 1. The other gear would connect via MIDI thru's. (Example: US-428 MIDI Out 2 to K1 in, K1 thru to Roland in, Roland thru etc....) Of course, all devices would need to be set to different MIDI channels. You would not really need the MIDISport.

I don't really understand what you mean by "do I use the US-428 as the master?".

In light of the fact that you're new to all this, my first suggestion would be to do a lot of reading (and be VERY patient with yourself...there's a lot to learn). Start with the Tascam site....I've written a number of docs there that should get you started on 428-specific stuff. Make sure you download the version three manual, as it's far more detailed than the one in the box.

Also, do some reading on basic MIDI and computer audio...there are a number of sites out there dedicated to this.

Once you start to get a basic understanding, you'll have more specific questions (which will therefore be easier to answer).

DK