according to this the word ZENZIZENZIZENZIC is no longer used, that is why it doesn't appear in the oxford english dictionary

Strictly speaking, that's why it only appears in the OED and not, for instance, the Shorter OED. The OED (I've got the Compact edition) contains every word in English* except those which became obsolete before 1150.

Peter

* Apart from "snog", which somehow got left out.