It's both. It comes down to the keyword identification algorithm of course, not the design by Google. Google's decision choice was not picking a random name that can be identified but instead working in their brand name. My bet is that give it a few years and we will have custom activation words within some constraints.

Is it both? The Google Home I was playing with at my in-laws house a couple days ago definitely only seemed to respond to "OK" (people kept forgetting and using "hey", which didn't work).

Works for me, though I've trained my voice on it, so perhaps that helps. I'm 100% it's supposed to be both.

When you're setting up the Home during the training step it makes you say "Hey Google" and "Okay Google". It's definitely a bit hard to notice considering only a single word changes.