I can usually come up with a way to practice just about any situation that I expect to face during an O' race. But, I can't think of a way to prepare for facing off against a dog. It is a good thing I don't have to worry about dogs at the races I'm running this weekend...at least I hope I don't have to worry about dogs...
The famous runner James Fixx wrote in his book about running that the best way to handle aggressive dogs is not to run away but run towards the dog. When you are close to the dog you should jump high and scream all you can. I think he called it "The big dogleap" or something like that.
I have never had the opportunity to try that method.