The source code can be downloaded from http://www.eso.org/chummel/oyster/oyster.html. It contains C and FORTRAN code which needs to be compiled, and regular IDL procedures.
Download files oyster.tar.gz, catalogs.tar.gz and atmospheres.tar.gz and unpack OYSTER first (e.g. tar zxf oyster.tar.gz). Then unpack the other two inside the newly created oyster home folder. Two directories are created, atmospheres and catalogs.