You know how you can say both "OK Google" and "Hey Google" to Google Home and it will respond to the two hotwords without any discrimination? The same isn't true of Assistant on your Android phone: it only answers to "OK Google." Wait, scratch that, it wasn't true as the command is now working on phones too.
Alright, it's not live for everyone just yet. For example, it was working flawlessly for me yesterday on my S7 Edge, now it doesn't want to answer me. Maybe it got mad because I've quasi-switched to a Nokia 8. And for the record, it doesn't work on the Nokia 8 either. But it's working on and off on my friend's S8. And we've been receiving several tips of it working for some users, though it's not nearly universal.
A few days ago, XDA unearthed signs of it in the Google app, so this wasn't unexpected. However, your mileage may really vary in this case. Try it out and let us know if it's working for you. Otherwise, I guess we'll have to wait for an announcement and Google to flip the switch for everyone.
Apparently, Assistant will prompt you to re-train itself for both "Hey Google" and "OK Google" variations (thanks, Logan):