One thing I don't understand about Forex, I'm trading a demo account on Oanda.

If my account is in CAD dollars, I then submit a buy order for EURUSD, how does this work? Does my CAD get converted to EUR which are then used to buy USD? Isn't that playing 2 different pairs at the same time? So if EURUSD goes up, but USDCAD goes down, I could actually lose money