Hallo Joseph,

first, you must find out, how high maximum voltage at DC line is, when ringing occurs. This voltage should NOT turn-on the transzorb, when transzorb is only used for supressing ESD energy.
Then, choose a transzorb with a threshold voltage of about twice this maximum voltage, for a start. A 1.5KEXX type should be well suited. Connect it from DC line to ground at your opto circuit.

Have also a look at www.st.com, for example. There are lots of good application notes, dealing with transzorbs and how to choose the right type.

As I meantioned earlier, it can be a good idea, to parallel a capacitor of about 10nF. But keep in mind, that with switched inductive loads heavy resonances can occur! So, careful investigation is to be done by yourself.

Don't worry about ESD events represented by human body model. Here, current limiting resistance is 1500 Ohm. Air gap model assumes about 330 Ohm and is more critical. But highest danger is expected with machine model, because there's no current limiting resistance at all! Happily, normal unidirectional transzorb is acting in the sub nano second range and can withstand more than 1500A for a duration of less than 1msec. So, it's a really good measure against ESD damage, especially when paralleled by some capacitance.

Kai