With the lack of current documentation, the best way to learn how to use Osiris is to look at other scripts, such as those provided by the base game (need to unpack Shared.pak to get to these).
For syntax, a lot of references are on the wiki:
https://docs.larian.game/Category:Osiris_APIs, and Norbyte posted a large list of function calls here:
http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=602683#Post602683, though be aware that not all of these are currently accurate, since some stuff changed between back then and now.
