One of the displays shown in post #2 turned up in the post today so I have had a look at why it does not work.
The I2C address is 0x3C but the OLED board does not acknowledge the I2C transfers.
The problem is that the board has been incorrectly fitted with uni-directional buffers and so will never work correctly. This is a design/manufacturing error!
My solution is to take the buffers U3 and U4 off the board and add wire links to replace the buffers. There are pullups on the display board (R2 and R3) already so these now do their job and extra resistors are not needed.
This picture shows the hacked area of the board:
Not a job for the faint hearted or those without a fine tip on their soldering iron!
After the hack the display works fine and shows up on an I2C scanner.
If you cannot do this hack then the board is scrap. The only other work-arounds I could suggest is to hack the Wire library to ignore the absence of the acknowledge or arrange it so another I2C function does the Ack for the display.
So my advice is do not buy these particular 128x32 I2C displays (see post #2)!