Against is a preposition. Oppose is a verb. Opposed [to] is an adjective. The other commenters have done a good job of explaining their difference. It just depends on the situation. Against literally means "In opposition to" in some circumstances. The dog goes against his owners commands. The dog opposes his owners commands. Both mean: The dog will not comply with his owners commands. The dog refuses to listen to his owners commands.

They have similar meanings, but different in how you say them. If you have in idea that I don't like, I would say: "I oppose your idea" or "I am against your idea" I could also say, "I am opposed to your idea" but I can't say "I against your idea"

"I'm opposed to that idea." "I'm against that idea." In both cases, the meaning is more or less the same, but we need "to" after "opposed". Also, "opposed to" has a formal register. For physical positions, you can only use "against". "He was leaning against the wall" makes sense, but "he was leaning opposed to the wall" sounds silly.