Can OADM drop port as well add?

– In short: Yes it can!

– If corresponding wavelength is injected into drop port, then it is added to signal. OADM can work from both sides. There is no essential differences between Add and Drop. Directions of light are reversible. For example, if we take look at scheme from EDGE Optics product… Then we see that any arrow direction can be reversed and everything will work. It is matter of equipment labelling.