Hi,
If you have Origin installed, origin.h file is located in this folder (assuming you have Origin 2018 installed):
C:\Program Files\OriginLab\Origin2018\OriginC\system
However, unless you are actually writing Origin C code to run within Origin, it isn't very useful.
Really, it is a series of #includes for other header files variously the following folder. They too are only useful for creating Origin C code
C:\Program Files\OriginLab\Origin2018\OriginC
I hope this helps.
Thanks,
Chris Drozdowski
Originlab Technical Support