Wow, there's so much wrong going on here. First of all, Orchard is a LEGEND.
Orchard isn't legendary.
Next, Oath will NOT trigger unless you have fewer creatures, and it will only resolve if you still have fewer creatures. You can't tap orchard during your upkeep to activate oath.
I'm not a judge, so I won't comment on this, but lets assume for a moment that the game plays out like this. How exactly does blocking the colossus and taking six damage cause a lock? Sure, you can tap his stuff and prevent untapping next turn, but you'll never win. Next turn you'll just oath up another dragon, then do...nothing. He untaps, attacks again. This will just repeat until he kills you with the colossus. You can never attack either, since waiting a turn for summoning sickness gives your opponent the chance to untap.
I suppose you could eventually win by playing a diamond valley, then stalling long enough to get a ridiculous amount of life and win from there, but that takes far too long. Why not just use a few mindslavers and white bringers? Such a lock is much more effective, as your opponent can't do ANYTHING after the bringer becomes active.