What is the right possibility ?
- I am opposite the wall
- I am opposite to the wall
- I am in the opposite to the wall
In general, to be opposite an object in the physical sense, there has to be something between you, and your face has to be looking toward a face of the object, at least figuratively.
The word opposite comes from the Latin oppono, but it appears in various forms in the European languages that Latin has influenced. The Wiktionary entry seems to lead in all directions.
To my ears, the first sentence sounds more natural, but so long as the core sense of opposition from the original Latin is maintained, you could probably use any number of prepositions to convey additional meaning.