The OnePlus 7 (or whatever it'll be called) will not be the company's first 5G phone keeping in mind the high costs of the technology. However, OnePlus is indeed testing 5G in Qualcomm's labs for incorporation in a possible new premium lineup. This could mean that we might see three smartphones from OnePlus next year.
https://www.notebookcheck.net/The-OnePlus-7-will-not-support-5G-but-the-company-has-plans-for-a-new-premium-lineup.360621.0.html