Conventional phase detection autofocus (PDAF), as featured on most modern smartphone cameras, works by detecting pattern changes in the vertical direction only. In other words, PDAF wouldn't work as well on horizontal pattern changes.
This is where "All Pixel Omni-directional PDAF" comes in. In a nutshell, it's about detecting pattern changes in both vertical and horizontal directions to make use of more image data, thus enabling faster focusing and better low-light performance. This also allows the sensor to focus on smaller objects or finer details.
All Pixel Omni-directional PDAF requires some tweaking on the sensor's micro lens level. To achieve this, Oppo tapped into Sony's "2x2 On-Chip Lens (OCL)" solution, which places four adjacent pixels with the same color under one on-chip lens, thus turning all imaging pixels into usable pixels for phase detection. A conventional sensor layout, on the other hand, would have one lens per pixel, which vastly limits the number of detection pixels.
Of course, this sounds great and all on paper, but we'll have to see it to believe it. Oppo didn't have performance figures to share at the time of writing, nor could it share a model number for its custom-made sensor. Likewise, there's no word on whether the under-screen camera will make it to this phone -- the prototype I tried today certainly wasn't ready, as the sample shots had a fair amount of hazing. We'll know more when the Find X2 launches in Q1 2020 -- maybe at MWC?