Does your Oneplus have a USB C port? I have a Oneplus 6 which does. Connect it to your Mac, open the Notifications area on top of the phone screen and change USB function from 'Charging' to 'File Transfer' and if you install and run the Android File Transfer app you can see all the stuff on the phone. What you see is a bit of a mess, but some important folders are:
Thanks Tamar. Mine is a Huawei Honor 8x but it gives the same message. I decided just to do a Select All > Copy from phone to desktop as I didn't know if any android software would just copy over what THEY thought was important.
I think it probably needs a Backup of system to enable a move to a new phone or a reset one but again, I wasn't sure that would back up everything.
Belts n braces!