OnePlus has updated its Switch app to add support for transferring data from iPhones. Unfortunately, there is no companion app on iOS on the App Store, so the process is a bit complicated.
The full set of instructions are on the OnePlus forum but the gist is that you need to download a beta version of the OnePlus Switch app for Android along with the iOS app from OnePlus website. Then you have to give special permissions to the app you just downloaded on your iPhone for it to work, since it is not an App Store app.
After that you get hotspot settings on your OnePlus phone, which you have to connect to on your iPhone. After that you can open the Switch app on your iPhone to begin the data transfer.
It’s not the easiest of processes and there’s some irony in welcoming iPhone users to your OnePlus ecosystem by giving a complicated set of tasks as the very first thing they have to do to get their data across on their new OnePlus phone.
Also, even though OnePlus claims iPhone transfer support in the update log for the Android app, the current version on the Play Store (2.1.2) doesn’t actually support this feature and only includes instructions to download the beta from OnePlus website (2.2.0), along with the separate iOS app, to be able to transfer the data.
But, if you have some important data stuck on your iPhone that you need to transfer, then at least now you have a way to do so.