Corba interoperability at the programming language level is based on IDL. The CORBA specification includes IDL into these development languages. CORBA will interoperate and seamlessly hide the difference between operating systems. Support for all operating systems is not guaranteed by the CORBA specification though. It is only the ORB vendors that decide what OS their products support.
Starting with version two of the CORBA specification, a common protocol for CORBA has been defined.
The Internet Inter-ORB Protocol (IIOP) provides a standard, on-the-wire protocol for CORBA, much like HTTP for web servers.
The IIOP also brings ORB-level interoperability to CORBA. Because IIOP is a well defined protocol, full interoperability between ORB products from different vendors is possible.
Realizing that an interoperable framework could not stop with the domain of each vendor, the OMG specified GIOP and IIOP so that all ORB prodcuts could speak a common low-level language, at this point ORB interoperability is assured.
