When BBK, the parent of Oppo, came up with the OnePlus phone, pretty much anyone outside of China never heard of Oppo.
Then they have Carl Pei develop a "start up" that is so pour, they won't be able to produce enough phones, so you need
an invite to buy one. Come up with a phone that was nothing more than a knock off of the Oppo Find 7, minus a feature here
and there, sell it DIRT CHEAP, call it a flagship killer and it created a feeding frenzy online. (remember?).
By the time it was discovered that OnePlus was owned by Oppo/BBK electronics, the phone had taken off and they released
another model, then another model building a base of users OUTSIDE of China. Of course they increased the price to the point
now it's not worth being a "flagship" killer for what you get. I had 3 of their phones. OnePlus 1, 5, 7 but after that, I switched.
Price, the OS, updates (lack of)...
From a budget standpoint, since Oppo is more well known, makes sense to drop OnePlus IF people will switch to the Oppo.
That also means figuring out a way to get approval (I'm talking USA here) from the FCC to get them to function properly on the
cellular networks.
The other problem, is since the Apple/Samsung duopoly probably has deals with the the carriers/government/politicians to KEEP
out the competition, and carriers probably won't allow outsiders on their networks, unless you know how to wrangle some of these
phones, they may not work on the networks.
And, since a LOT of American consumers still continue to "buy on contract" with "free upgrades", walk into a carrier store and you pretty
much see nothing buy Apple, Samsung, Google, Motorola and maybe one or two others.