[NTG-context] small caps and old numbers with open type fonts

Hello list, I'm still playing with a few otf fonts and enjoying the way ConTeXt works :) I'm now playing with a beautiful Garamond. I try to typeset a few things with old-style numbers and, well, the output contains old-style numbers but not in my Garamond (it seems to be in latin modern). What should I do to use the old-style numbers of my font? Did a miss a doc about that? Or did I missunderstood the \os command (I thought it was a switch to the old-style of the font but it seems rather to change of font)? I actually use my font as simple as possible with a \definefont[boum][MyFont at 12pt]. Is it the problem? It the same with the \sc command. Thanks a lot for your reply, -AJ