I thought this was a very interesting question because I'm struggling with this issue right now. I'm trying to decide whether I can live without some of the other keyboards I am using now that I have the Oasys (my band members would really love me to be using less space).
The Oasys will be replacing my Kurzweil SP2X (which I just listed on ebay) but I have three other keyboards and I am not convinced that ANY of them are replaceable. It's not really a question of broadening my sound palette. If that was my main concern, then a few controllers hooked up either to a laptop with MainStage and soft synths or to a Muse Receptor would provide for every contingency.
The main reason I bought the Oasys was for its playability --- it's definitely an inspirational instrument, both in sounds and in its playability. There are very few electronic instruments for which that's completely true. But there are some:
I also play a Minimoog --- and even if the LAC could really match it for sound, (which it doesn't, but it does have its own niche), you don't get the playability that comes from the lighter keyboard and the direct hand-on control of all parameters - there's just nothing like turning those knobs!
Then there's the Hammond. Between dual manuals, waterfall keys, and the drawbars, you can't REALLY play hammond style on the Oasys, even though it's got a darn good tonewheel implementation. Again, it's not JUST about the sound.
I was very interested to read the interview with Tony Banks on how he used the Oasys on his last tour. I did go to one of those concerts and noticed he had an Oasys but I didn't know much about it at the time. I read however that he was using it as his main engine with just a few other controllers hooked into it. I thought that was interesting but I did wonder if he would have been happy with that setup if he was playing a lot of the good stuff that they did just before Gabriel left.
David
Just out of curiosity, what is it about the Oasys that makes you feel you need a Synthesizer (specifically) to accompany it ? Is is simply a case of broadening your sound palette ?