generic 128x64 graphic OLED displays on eBay to play with. It took some trial and error to figure out how to get the Arduino Pro Micro to talk to them, and there's lots of different information on the net about doing so (probably owing to the wide variety of implementations of these displays and generic controller chips.)
This particular one uses the SD1306 controller and is wired for a 4-wire SPI interface.
I used the u8glib library to talk to it. The "gallery" page on the library repo provides lots of examples of people's various combinations to make these work. This particular one wasn't shown, but by perusing through the different examples I was able to find a combination that worked.
I'm using the SW SPI function of u8glib, so I don't have to use the ARM chip's native SCK or MOSI pins. I found that this device will accept data without the CS pin connected (pulled low instead), so I left that out to save an output pin. (You'd want to use CS if you were trying to use the same SW SPI bus to talk to multiple devices.)
I wired it as shown:
Arduino Pin -> OLED pin
15 ----> SCK
14 ----> SDA
16 ----> RES
10 ----> DC
Then, in the u8glib "Hello, World" example, I used the following definition:
// SW SPI Com: SCK = 15, MOSI = 14, CS = N/C, A0 = 10, RESET = 16
U8GLIB_SSD1306_128X64 u8g(15, 14, U8G_PIN_NONE, 10, 16);
Note: The "RESET" pin was the crucial one I was missing in my initial attempts, as it's not defined in the SD1306 example configs in the source. I had even tried tying the RES line high or low to no avail.
With those four lines, though, it works fine! I'm not sure what the eBay auction page means by "only needs two I/O pins from Arduino." Maybe they copied that from the I2C version. :-P