Everything damages the battery (charging it, heat, cold) and so on. Normally, dash charging SHOULD NOT damage your battery life. However, I have an OnePlus 3T, which I bought 4 January 2017. And after one year, my battery life is pretty much the same. Honestly, I think on android phones the most important thing is the OS you are running and the apps you installed. I am very happy with oxygen OS. I once installed other roms and I even rooted it, but my bank app wouldn't open anymore because I had a costum rom and root. That's why I've changed back but I am very happy. So in my case, I have to be careful what apps I install.
What I try to say is, if you want to have the best battery experience on the one plus, always keep an eye on the most recent app you've installed.