Yes you need a nano SIM for the OnePlus phone.
Check your existing SIM in case the nano SIM can be popped out of the micro SIM.
If not then you could try getting it cut in a phone shop.
But it would probably be safer to just order a replacement trio SIM and then popping the nano SIM out of that.
If you to that then make sure to choose the "swap" rather than "block" option so that your existing micro SIM doesn't get blocked immediately and you can continue to use it in the meantime.
This key post is also of relevance here:
Hope this helps.
You activate the replacement SIM when you get it.
The links that I posted above explain the whole process.