how to change the update server

I want it to this http://gentoo.osuosl.org

Sorry, but http://gentoo.osuosl.org not a calculate mirror.
Look, this work same way:

cl-update get updates (portage, binary packages) from calculate mirrors. Its reasonable, for consistent portage/binary packages system. It ruled by variable update.cl_update_binhost_list as Alexander say.

if no binary package or you restrict binary packages(for why?) - soft builded from portages an get distfiles getoo-way (GENTOO_MIRRORS=http://gentoo.osuosl.org as written above)

If I understand U correct, all question answered:

  • for gentoo distfiles u may use http://gentoo.osuosl.org/, but calculate binary packages will download from calculate mirror
  • for calculate binary packages used only calculate mirrors (not gentoo, not gentoo.osuosl.org)