What is the relation between Reconfigurable Optical Add/Drop Multiplexer and Optical Cross-Connect? Are they two different things technically? or they are two concepts which have many things in common?
An optical cross-connect is a more general term for an optical switching device. The switching can be done electronically, purely optically, or mixed.
An optical add-drop multiplexer is a more specific device for wavelength-division multiplexing (WDM) that can merge and split off specific wavelengths/colors. With a reconfigurable OADM that behavior can be remotely controlled.