Changing to a new phone can be a hassle; it's especially annoying for those of us who have to do it often. Thankfully, many OEMs include a proprietary data transfer app to help you through the process. OnePlus has the Switch app, and it's just been updated with support for moving from an iPhone.
The latest version (188.8.131.52105142054.86aba58) enables iPhone data transfer, although the feature is still in beta, so be warned that there may be some teething issues. It was introduced on the OnePlus forum at the end of last week. You'll need this app downloaded on your iPhone, which is a test app and therefore not yet available through the App Store.
Go to the Switch app on your OnePlus phone and it'll walk you through the process. I don't have an iPhone myself (as that would be blasphemous) so I can't test it out. If you do give it a go, let us know in the comments how you found the experience.
This latest update to OnePlus Switch should be rolling out widely via the Play Store right now. You can also download it from APK Mirror if you don't want to wait.