General Synonyms:
* Opposition
* Hostility
* Conflict
* Friction
* Contention
* Enmity
* Animosity
* Aversion
* Resistance
* Discord
More specific synonyms:
* Adverseness (antagonism towards something)
* Antipathy (a strong dislike)
* Repugnance (strong aversion)
* Feud (a prolonged and bitter conflict)
* Rivalry (competition for the same thing)
The best synonym for "antagonism" will depend on the context of your writing. Consider the specific situation and the emotional intensity you want to convey.