The Gathering for Open Science Hardware (GOSH) initiative aims to propagate the use of scientific tools build as Open Hardware. "Open Science Hardware" as they call it subsumes any Open Hardware used to conduct scientific research. Your question can therefore be answered about Open (Source) Hardware in general, no matter if used for science or not.
With Open Hardware all plans and software are available and can freely be used to rebuild the hardware from scratch. At its best all parts are also openly described and unrestricted. Still it is possible to buy parts from commercial vendors but these parts should also be open or at least specified in a way that they could easily be replaced by parts from other vendors (e.g. industry standards for basic mechanical and electrical parts).