The Pro variant is OnePlus' newest marketing tag, and will be used for the first time to differentiate one variant of the upcoming 7 flagship from another. DisplayMate has announced that it has given this phone its "highest A+" rating. Therefore, the OnePlus 7 Pro now goes into its May 2019 launch under this additional expectation.
https://www.notebookcheck.net/OnePlus-7-Pro-scores-DisplayMate-s-highest-accolade-even-prior-to-its-launch.419771.0.html
Well they've refreshing the "highest A+" for quite a few years now so it shouldn't matter between those models, but there still is a notable difference in OLED panel quality among those not reviewed by Displaymate.