OnePlus doesn't say a lot about its phone sales. You might get the occasional empty platitude about sales exceeding expectations or what have you, but CEO Pete Lau reportedly offered a real sales metric to PCMag's Sascha Segan. He says sales of the 6T have jumped 249% compared to the OnePlus 6 launched earlier this year.
There are several differences between the 6 and 6T. The newer phone has a smaller teardrop notch, an in-display fingerprint sensor, and no headphone jack. None of that was likely to move the needle very much. The real reason for OP's massive sales boost is the T-Mobile partnership. The OnePlus 6T is the first phone from the company sold directly by a US carrier, which gets it in front of more consumers who had previously never heard of OnePlus. It would appear the lack of a headphone jack has not upset the OnePlus apple cart as many predicted.
Selling an unlocked phone is all well and good, but there are only so many people who know they can go online and drop $550 on a pretty good phone. By contrast, a carrier handles setup and finances your phone so you can pay it off over two years. The 6T adds $24.17 to the monthly bill, a bit less than more expensive phones. And I bet the in-display fingerprint sensor demos really well in stores. OnePlus might see more carrier successes in the future—it's aiming to get in on 5G early, which could mean stronger partnerships.