OTS=One Touch Setting.
On the PSR740, 2000, 9000 and 9000pro (and maybe some others I am not familiar with)each style gives you 4 buttons of voices recommended for that style. On the 9K and Pro, you can change the voices to your taste. On the 2000, no need to change them because there are 8 Registration buttons where you can store your favorites. And if those 12 aren't enough, the 2000 lets you store multiple banks of 8. Really a cool feature.