RKA

So when you flushed it with cold water, some of the oil came out, and when you drove the car, more oil started to mix with the coolant.

I say keep flushing the cooling system, even better, use a detergent designed for this, and flush well with water afterwards.

If you're convinced it's the head gasket, do the leak down test. Shouldn't cost you more than an hour of labor, but I really think there would be other symptoms (white smoke out the tailpipe) if you had a bad head gasket leak.

computrtutr

Is there anything on the underside of the oil fill cap? What kind of coolant are you using (i.e. what color?)You could drain the oil into a clear container and let it sit for an hour- Look at the side of the container for a line where the coolant and oil separate? Conversley, it might be more telling to do this by draining your radiator of coolant (instead of draining the oil from the engine- Look for separation, which will tell you if anything is mixing. Are you throwing any codes? A VAG scan might be of help. How much does your exhaust drip (with the AC off)? Are there any coolant or oil stains on your heads or block? Have you noticed any excessive fluid consumption?
Does your dipstick have any fluid in addition to oil on it? Have you used any additives (i.e. stop-leak) that might have not mixed/emulsified properly?
I don't know how useful these things will be to you, but it's where I would start.