Hello Zalman
The Oasys is basically a workstation rather then an arranger and is essentially a computer and keyboard in one case, with a program written for music, (Just like Word was written for Word Processing) however there are two other manufactures that also have a computer and keyboard in a case, and these are arrangers, one is made by Wersi and is called the Ikarus and the other is by Lionstracs and called the Media station.
As they are all computer based it means upgrading is easy (You don’t need to change the keyboard) and in most cases you are not tied to what the manufacture gives you. (You can use VST plug-ins, Gigasamples etc)
Hope this helps.

Bill