...X environment on micro core (and may not be the FLTK)? I've installed openbox, but startx doesn't work.
If I understand it correctly, openbox is a window manager but you won't have any windows to manage until you install an X server.
Try loading Xlibs.tcz, Xprogs.tcz and either Xvesa.tcz or one of the Xorg-7.<X>.tcz extensions in addition to openbox.tcz.
Edit (21:37): You might also want to load wbar.tcz - I don't know if open box has some alternative app launcher or not. In any case, Micro Core + XLibs + Xprogs + Xvesa + wbar + flwm_topside = Tiny Core so if your goal is to substitute open box for flwm_topside, its that easy.