There are multitude of OS & software in running. Some people still use DOS, but most of those were already upgraded - to windows XP. These machines are currently being replaced with Win10 ones. But due to some specific old SW there still need to be some DOS machines running, at least for couple upcoming years. Linux is sadly not an option for typical office workers, again due to some software in use. There's at least open source in places where possible with more (Firefox, Thunderbird, tightVNC, ...) or less (LibreOffice) success.