The next bus example uses this pin connection scheme (copied and pasted) and I added the pin descriptions in italics:
Wiring (Photon ⇒ OLED screen):
D4 ⇒ CS (Chip Select)
D3 ⇒ DC (Data/Command Control)
D5 ⇒ RST (Reset)
A5 ⇒ D1 (SPI MOSI)
A3 ⇒ D0 (SPI SCK or Clock)
3V3 ⇒ VCC (3.3V Supply)
GND ⇒ GND (Ground)
It is important to understand the pins and what they do. The chip select pin either enables or disables the display. If multiple screens were communicating on the same SPI bus, then you use chip select to turn on the desired screen and turn off the others as you can only communicate with one device at a time (the display library handles this for you). The data/command control pin tells the display if you want to update the data registers or the command registers (the display library handles this for you.) Reset is a bit self explanatory. I know the example is using SPI because the A3 and A5 pins on the Photon are for SPI communication, not I2C.
If you choose another display based on SSD1306, then D1 and D0 might be labeled MOSI and CLK, respectively, or some variation of those labels. Don’t get hung up on the labels when shopping. Understand the pins, read the datasheets!