Google it. The more people use it, the more likely it is to be correct. Right? :-)

If you google for "opposite of synergy", you get several suggestions, including "antagonism" and "antergy". "Antagonism" seems to me to be not quite right, and "antergy" is downright obscure. Also "dissynergy" looks like an unnatural construction.

So I would stick with the phrase "opposite of synergy" because its meaning is perfectly clear, and it is not inelegant.

Disergie

Eine von mehreren Definition aus dem Webster's Third New International Dictionary:

antagonism:
2b: interaction of two or more substances such that the action of any one of them on living cells or tissues is modified (as by interference with the uptake or by an opposing physiological rection) - opposed to synergism

The opposite of synergy is disergy which is the result of two business merging and creating more bureaucrazy making both the businesses less effcient.

The term antagonism generally means "the opposite of something". So this cannot be used in this context.

"The opposite of synergy" sounds a bit awkward if you are talking about the "synergies/dissynergies of a merger of companies".

Google only shows about 600 results for all kinds of spellings, all pages from Malaysia, France, or other non-native countries.

....


still need your help

http://www.brockhaus.de/wissen/dyssynergie:

Quelle: Der Brockhaus Gesundheit

Dyssynergie: Störung des Zusammenwirkens zusammengehörender Funktionseinheiten.

Der Duden kennt auch Dysergie auch im Kontext Gesundheit, allerdings mit andere Bedeutung