Adafruit gives a image tutorial on how to level shift the arduino's 5V logic to an SPI OLED screen's 3.3V logic. Adafruit provides a Ch4050 level shifter to make this happen. To me it would make sense for arduino's 5V logic to enter one side of the shifter and leave the other side as 3.3V logic to the screen. But that is not what the wiring looks like from the looks of this picture.
How do I understand how to wire my Arduino to the Ch4050 to the OLED?
It's hard to comment on the wiring diagram without any information about how the Arduino pins are set up and what the chip pins relate to - however don't get too hung-up on arduino outputs entering one 'side' of the level shifter and coming out of the 'other side'. Could you Google for the datasheet for the level shifter chip and see how the pins are set up? That should help you determine whether you have wired up the chip's inputs (from Arduino) and outputs (to OLED) correctly.
The pinout isn't what you think it is. The outputs are right next to their corresponding input, rather than the inputs being on one side and the outputs on the other. This is very clear if you look at the datasheet.
You should always pull the datasheet for parts you're considering working with; datasheets are the electronics designer's bible - like the big bible, they contain truth and wisdom from the creator [of the part] in an obtuse and confusing writing style, and you would do well to heed their lessons, else you will end up dealing with fire and brimstone [or, er, usually just smoke]. Some even have inspiring anecdotes [app notes] that show how to apply these lessons!
If you are only writing to the OLED, and never sending data back from the 3,3 to the 5 v. side, all you need is voltage dividers for each connection from the 5 v. to the 3.3 v. side. I've done that, works fine.
