Here's a great website (for which one of the owners has joined here recently queenvault ) which lists a more or less definitive list of Pre-Queen and Queen + and solo projects. Plus all of their various edits and such.
Although we know of an acetate containing 3 tracks by The Opposition, only one of the songs was actually composed by them, and it's more of an improv. So no real document of Deaky having written anything pre-Queen as far as I know.
I have just had a quick look on the queenvault and had forgotten The Opposition had also be known as Art. Looks like they might just have been a cover band.
One thing confuses me about the short bio on that site, though - it states the acetate was recorded in 1970, and the band featured Deacon. However it then goes on to say "later in 1969, John Deacon left for London". Well, 1969 is certainly not later than 1970 lol, so....a number of scenarios here:
1 - the 1969 part is a typo 2 - the acetate wasn't recorded in 1970 3 - it WAS recorded in 1970, and Deaky, who had left for London in 1969, returned home to record it 4 - it WAS recorded in 1970, and Deaky isn't on it at all....
Deaky was not part of the ART/OPPOSITION from 1969, so it is true that the acetate was recorded the following year. However, various sources claim that the was asked to perform on the recording, so he is involved in it. I believe this is true.