The incorrect plural noun is oxes. The correct plural form of "ox" is oxen.

Here are the incorrect plural nouns in each set:

1. The incorrect plural noun is oxes. The correct plural form of "ox" is oxen.
2. The incorrect plural noun is witchs. The correct plural form of "witch" is witches.
3. The incorrect plural noun is knifes. The correct plural form of "knife" is knives.
4. The incorrect plural noun is duckes. The correct plural form of "duck" is ducks.
5. The incorrect plural noun is advices. "Advice" is an uncountable noun and does not have a plural form.