Because it is very old and probably not documented. Before OASIS was called OCamlAutobuild. And _oasis was the name of the toplevel file.
It stands for
OCaml Autobuild Source Information System 

Don't ask the real meaning, I just wanted something that will be right for a filename (_oasis) and related to the desert...

https://forge.ocamlcore.org/docman/view.php/54/94/SlidesCamlConsortium2009.pdf

During this precise consortium session, we decided that OCamlAutobuild was not a right name and change it to OASIS...