"oppose and object "is verb, but against is 介词，

against 也许可以是“反对”的，（He is against(opposed to) my decision) 。 你可以说“I am against什么什么”还是“I oppose什么什么”还有“I object to什么什么”都是一样的意思。

oppose/object = 反对，不赞程。。（He opposed my decision) against = 也是反对，不过后面一定要有人，事或物。（He fought against his enemy)