You say that two ideas or systems are opposed when they are opposite to each other or very different from each other. people with policies almost diametrically opposed to his own This was a straight conflict of directly opposed aims. = opposite
You use as opposed to when you want to make it clear that you are talking about one particular thing and not something else. We ate in the restaurant, as opposed to the bistro
If you are opposed to something, you disagree with it or disapprove of it. I am utterly opposed to any form of terrorism = against