OLED displays don't inherently have poor text rendering, but they tend to use more exotic subpixel layouts than LCD displays [0]. It's possible to render text well with alternative subpixel layouts (for example, every iPhone), but if you use the standard RGB/BGR algorithm with PenTile displays, you're going to get bad results (but with a high enough resolution, it doesn't really make much of a difference).

[0]: https://en.wikipedia.org/wiki/PenTile_matrix_family