I have two modules connected to an Arduino Pro Mini over SPI, an SSD1306 128x64 display and a nrf24L01+ wireless module. Individually, both work wonderfully. With both on the hardware SPI bus the nrf24L01+ seems to work fine, but the SSD1306 display is useless except for displaying garbage. I've tried several different sketches to try to tease out what is wrong. If a sketch sends any information via the nrf24L01+, and therefore the SPI bus, garbage results on the display. The nrf24L01+ is connected to the Arduino with SCK->13, MOSI->11, MISO->12), the nrf24L01+ with CS->7 and CE->7, the SSD1306 with CS->10 and A0->9. (I'm not really sure what A0 is.)
Doing a bit of reading on the Arduino implementation of SPI, it looks like I may have to use software SPI for this. Sound about right?
Update: I tried using the same configuration (didn't move any wires around) with software SPI. I get approximately the same affect. It seems like the display doesn't honour the CS signal, and treats all data sent over the SPI bus as something it should act upon.