Thanks to AdaFruit for their libraries. However, my clone OLED does not have a Reset pin, so I had to improvise...
I powered the OLED from Arduino pin D2, allowing me to turn it on just before the library start sending I2C commands. Without this reset/power control, all I ever saw on the OLED were random dots. :-(
Thanks to Nick Gammon for his I2C Scanner. That sketch told me that my OLED I2C address is 0x3C and not 0x78 indicated on the board.