MIDI control: I initially thought that MIDI control with the OP-1 was modal. That is the OP-1 only sends out MIDI notes while in MIDI mode. (Accessed by SHIFT+COM / 1 [CTRL]) This is not the case.
The MIDI control page can be use to set channel and knob settings and in this mode the note keys send out note on and off messages. However if you drop back into OP-1 mode the sequencer data is also sent out to the specified MIDI channel (or the default channel 1) so the OP-1 sequencers can control soft synths. The OP-1 synths will be playing at the same time so their channel output can be set to 0 in the Master page though the option is there to record OP-1 audio at the same time into your DAW.
Virus control: This method also works with the Access Virus when using the Live template provided by Access. Which is really great because I much prefer the Virus sounds to the OP-1. It does make the OP-1 an expensive controller but adding in the OP-1 synths will make a more layered sound.
There is one strange thing though. If I select channels 2 or 3 or 4 on the OP-1 these will access the correct chnnels in the Virus but will also send note events to channel 1. There are invisible notes because if the channel selected on the OP-1 is 2 then using MIDI Monitor only channel 2 notes are appearing as output from the OP-1. Channel 1 in Live also showed the notes. This was solved just as I was writing this post by unplugging the additional MIDI cable which went from the MOTU Ultralite audio interface to the Virus.
Crossbow or Gun?: The central feature of the OP-1 is the tape recorder which records (obviously) the audio but also has a lot of editing and tape manipulation features. It is well done however is it really the best way to put together a song? At this point in time I can make a lot of strange noises with the tape recordings but nothing really usable. I can see some use in creating some unique sounds but I am not really getting it for a song creation method.
Tip: It is mentioned lightly in the manual by putting the OP-1 into USB mode does allow you to create a folders under the Synthesizer and Drum folders into which you can drop you AIFF files. When the OP-1 is then set back to OP-1 mode these should be read by OP-1 for use by selecting them as a sound source.
I found that the OP-1 endless controller which has note length settings and the ability to specify long notes is the most useful of the sequencers with longer samples. Sort drum style samples and notes can work with any of the sequencers.