Compilation always includes the procedure code which is then saved in oyster/oyster.cpr. This does not apply to GDL installations, where code is compiled on the fly. Makefiles exists in the main directory which simplify the installation. Use make -f Makefile.idl to compile the IDL installation, and make -f Makefile.gdl to prepare the GDL installation.
If you need to compile the C/FORTRAN external library due to incompatibility of your OS with the pre-compiled Linux 64-bit version, unpack the src.tar.gz file (to be obtained from the author) in the oyster/source folder. Make sure your OS is either Linux or Darwin, checking with the uname UNIX command. The Makefiles will use uname to determine which of the pre-installed Makefiles in the library directories are used for compilation. The only part of the Makefile which still has to be edited manually before running make is Makefile.uname.in in source/c which needs the path to the local IDL installation, as well as the location of the libgfortran.a library file. Then run the make -f Makefile.src command in the home folder.