Best answer: Most people should buy their OnePlus 8 unlocked. You get the most options in terms of the multiple color and storage choices, and you can deal directly with OnePlus for warranty, support and updates. The only reason to buy from a carrier is if you're on Verizon, because that's the only way you can get Verizon 5G.
Should you buy your OnePlus 8 unlocked or from a carrier?
Here in the U.S., you have three options for buying the OnePlus 8: from OnePlus unlocked, from T-Mobile, or from Verizon. It should also be noted that you can only buy the higher-end OnePlus 8 Pro unlocked. No U.S. carriers will be selling it.
If you want to get Verizon 5G, you have to buy the phone from Verizon.
There's one big reason you would want to buy the OnePlus 8 from a carrier: network compatibility. This year, that means 5G. and the only carrier that applies to is Verizon because it's selling a specific OnePlus 8 that works with its 5G network — the unlocked OnePlus 8 is stuck on Verizon LTE. So if you're a Verizon customer and want 5G, you need to buy from Verizon.
Outside of that, we recommend buying the OnePlus 8 unlocked. The unlocked OnePlus 8 has support for all of the U.S. carriers, including T-Mobile's 5G network, even though T-Mobile also sells the phone directly. There are issues with 5G support on AT&T, but AT&T also isn't selling the phone anyway, so you're out of luck there regardless. As far as specs, features and network compatibilities go, there's no functional difference in using the unlocked phone and the phone bought from T-Mobile.
The default choice should be to buy unlocked, and only go to a carrier if you have a specific reason.
There's one last question to address: water resistance. Technically only the OnePlus 8 you buy from Verizon or T-Mobile has an IP68 water and dust resistance rating, which could give you some pause when considering an unlocked model. While OnePlus doesn't advertise that IP rating on its unlocked phone, it's well recognized that the unlocked and T-Mobile versions of the phone are the same, so it can easily be inferred that the unlocked model is also IP68 rated.
With all that said, the default choice for everyone should be to buy the OnePlus 8 unlocked. From there, you can consider going to a carrier for specific reasons, such as compatibility with Verizon's 5G network, or a particularly good sale from the carrier. Unless you have a strong reason to buy from a carrier, get the OnePlus 8 unlocked.
The best choice for most people
Buying unlocked from OnePlus is the best way to go, even if you plan to use the 8 on T-Mobile. You get the same network capabilities, plus you get to skip the carrier tie-in.
Sometimes you just want to stay on your carrier
The only way to get a OnePlus 8 with Verizon 5G is to buy directly from Verizon. It is otherwise identical to the unlocked model, aside from an exclusive silver color option.
