The bootloader unlocking experience on a T-Mobile-branded OnePlus smartphone is very much different from the unlocked variants. Once you pay off the contract and use the device on the network for at least 40 days, you’re eligible to perform a SIM unlock. After SIM unlocking, you need to generate a unique unlock token to be able to unlock the bootloader. Besides the added complexity of bootloader unlocking, the slower update frequency and the inability to participate in the OxygenOS Open Beta and Developer Preview programs are some of the main reasons why people prefer to "rebrand" their T-Mobile OnePlus phones to international (i.e. non-T-Mobile) firmware. If you have the T-Mobile OnePlus 7T and want to get rid of the carrier firmware, then you’ll be interested in learning that XDA Senior Member Superboy58 has managed to find a way to convert this variant to international without a traditionally unlocked bootloader/SIM unlock.
The trick is to patch the OPS file from the official unbrick package in such a way that the bootloader ignores the token generation part and allows users to unlock it with a standard Fastboot command. As soon as you unlock the bootloader, pick the Fastboot-flashable global firmware for the OnePlus 7T and do a clean flash to complete the rebranding process. Head over to the thread linked below for more instructions.
Rebrand T-Mobile OnePlus 7T to international firmware — XDA Thread
In case you have the T-Mobile OnePlus 7T Pro McLaren Edition, there's a similar mod to unlock the bootloader without waiting. The only catch is, you can't rebrand it like the other model. The T-Mobile variant is 5G capable, whereas an international variant of the OnePlus 7T Pro 5G simply does not exist. There is no way you can replace the stock T-Mobile McLaren Edition firmware with another official OxygenOS ROM. You can, however, opt for custom ROMs that are specifically built for this model.
Bootloader unlock T-Mobile OnePlus 7T Pro McLaren Edition without waiting