In addition to creating a favorable environment for its 5G-enabled phones to deliver considerably higher download speeds (eventually) than 4G LTE-only handsets, T-Mo obviously needs to make sure said phones are actually capable of leveraging all this super-advanced cellular technology from both a hardware and software standpoint.
For the time being, however, the "Un-carrier" only provides 5G service through bands n41, n71, N260, and N261, the latter two of which use millimeter wave technology to rival Verizon's insane high-band speeds "in pockets of select cities." Band 41 is where Sprint's 2.5 GHz mid-band spectrum comes from, while n71 is synonymous with the backbone of T-Mobile's 5G network right now, delivering a low-band signal that travels far but only marginally improves 4G LTE speeds.
In other words, T-Mobile's OnePlus 8 5G is being prepared for future 5G rollouts on additional bands after making its commercial debut with 5G support on bands 5, 41, and 71. In case you're wondering, no, this particular 6.55-inch handset with an 8GB RAM count, 4,300mAh battery, and 48 + 16 + 2MP triple rear-facing camera system doesn't have what it takes to support mmWave speeds from a hardware standpoint, so you shouldn't expect a software update to enable that.
Interestingly, the only other 5G phone sold by T-Mobile currently listed with 5G support for bands 2 and 66 is the LG V60 ThinQ, although we obviously expect Samsung's Galaxy S20 family to follow suit eventually, possibly alongside the Galaxy Note 10+ and OnePlus 7T Pro 5G McLaren as well.
For now, we should also mention the extremely well-reviewed OnePlus 8 5G is getting Google's live caption functionality, "general improvements and fixes", and April 2020 security patches too as part of the same 300MB+ update on T-Mobile.