While I despise op. cit (it's a horrible, medieval, tradition hostile to readers), I guess it can't hurt. But can the behavior actually be coded in CSL? Just any reference that is not the first?
From reading the thread, we can handle the required logic with <if position="subsequent"/>. The term would just reduce the number of "verbatims" we have in styles.
"Op. cit." is used in a lot of styles in use in France. And if "ibid" is already there, why not "op. cit." ?