With the T-Mobile and OnePlus partnership over with, there are no longer any USA carriers that offer OnePlus devices. Helium Mobile is a carrier that:

A) Costs way less that major carriers
B) Offers rewards that can be used to pay your phone bill or converted to USD

Add this with a monthly payment plan from OnePlus, and you might end up paying under $50 a month for a OnePlus phone and 5G cellular service.