Oppo will likely get design help from within China. Plus, it's not as if they're like K-Mart or something. They're a big electronics and telecommunications company with real world clout, lots of money, and design expertise.
Furthermore, they don't actually need to challenge Apple for performance, despite what press and PR and some geek forums might claim. They need something that's decent and cheap to be successful.
Remember, these companies' bread and butter isn't the bleeding edge. They operate on volume, with much of that volume at the mid and lower ranges.
Yes, but they need Snapdragon (or better than Snapdragon) performance at MediaTek prices, which might not be possible, especially if they’re aiming for volume. Too expensive, and it’ll hurt their ability to put out phones in large volume (by cutting into already fairly thin profit margins). Too weak, and they might as well be using off the shelf processors.
And stock Android is a lot like x86 Windows, vs iOS, which is more like an Intel Mac. Keeping close to stock Android makes it difficult to optimize for your custom processor. X86 Windows generally is expected to run reasonably well on any x86 chip, from Intel (including Atom), AMD, but even Via (and several others formerly), while Apple was able to optimize macOS performance for specific Intel microarchitectures. Likewise, Android is generally expected to work well across most, if not all, ARM microarchitectures, while iOS is able to influence Apple Silicon design, and iOS can be tweaked specifically to run the best it can on Apple Silicon. You could probably do that with a custom Android fork, but it would be nearly impossible to get into stock Android. (What happens if your patch for optimized performance on your custom SoC causes slower performance on other ARM SoCs?)
Besides, Android is a little like one of the truck chassis you can purchase from Ford. The chassis can be used to make a van, a full sized bus, or any number of vehicle types, it’s a flexible platform for building multiple large commercial vehicle types on. While iOS is more like a finely tuned supercar (or maybe more like a performant luxury car series, like BMW’s 5 series), since it doesn’t need (as much*) to be a generalized basis for other vehicles, while Android (and Linux itself, for that matter) is a framework for building different types of OSes on a multitude of hardware types (not even limited to ARM).
* Sure, Apple has some bare minimum of what is iOS that it can use as firmware in devices running A series chips (such as the new Studio Display) or as the basis for new OSes (such as tvOS or watchOS, both of which have a bare minimum version suitable for use in other devices, such as how the HomePod software is built on tvOS and the TouchBar was built on watchOS). But that bare minimum likely just consists of a customized version of Darwin that targets specific environments (macOS Darwin - full powered environments, iOS Darwin - general purpose environments, tvOS Darwin - minor resource constraints and older A series chips, watchOS Darwin - major resource constraints and very low power drain environments) and some basic userland that’s suitable for whatever custom UI the device needs (Obj-C/Swift, minimalist versions of Quartz**, Cocoa, etc.). And these bare minimums are still able to be optimized for specific pieces of Apple Silicon hardware.
** Devices without a screen may even be able to forego Quartz. It would be interesting to see how the internal architecture of the HomePod works.