Most display drive chips that work like a few common ancestor chips. Chip makers do this to make it easier for product manufacturers to migrate to their parts. A driver can be modified with little work. The descriptors include the size of the display in pixels, which in my case was 128 x 128 pixels.

Whereas LCD pixels require a voltage to control how much light they absorb, OLEDs require current to control how much light they emit. So the x-y grid has to carry significant current. The more rows of pixels, the shorter the time each row is pulsed on, and the higher the current in each pulse. This restricts the number of pixels a passive OLED display can support. One way round this is to have the OLED pixels constructed on top of a silicon chip. Silicon chips are fairly small, so such displays typically used in eyepieces where lens systems add to the total cost.

You can get OLED displays that are large (TV size), or tiny (eyepiece microdisplays), but there are applications for small displays that can be read without magnification. These involve small, battery-powered gadgets like smart watches. The display I got hold of was octagonal, to fit in round expensive hybrid mechanical smartwatches.

I can't completely see why a display needs to be round when one can easily make a watch body rectangular, but maybe watch dials look more natural on a round case.

I don't have an application for a small OLED display at the moment, but it was an interesting experience writing the drivers and getting it going. It often failed to recompile working code (with trivial mods), so if I had to do another project I would drive them with an FPGA and avoid the massive Linux OS. This would also be a better fit for battery-powered projects.