The GNU Data language (GDL) is "a free/libre/open source incremental compiler compatible with IDL and to some extent with PV-WAVE."
To install GDL on Code Ocean, use apt-get from within Code Ocean's custom package management system to install gnudatalanguage.
To see IDL in practice on Code Ocean, check out NOAA Ocean Surface Winds Team Cross-polarization Geophysical Model Function (IDL). Note that this capsule also installs idl-coyote via the postInstall script.